Add API documentation

This MR implements an auto-generated API documentation with drf-spectacular.

As the steering API uses a lot of subpath resources a few changes are necessary in order to make documentation feasible. As some of these changes are quite invasive (in the sense that they replace a lot of custom with generic logic) we should invest in a little bit of testing before this is merged.

Depending on the remaining time I can spent on this issue, I will implement these myself.

Possibly breaking changes in the external API:

• APIException objects are now converted to error lists in the form of [ { "message: "a message", "code": "an optional code" } ] making them easier to handle for client code (see c9164158). This also affects the hard errors raised during schedule creation/updates.
• Hard errors raised during schedule creation/updates now return a 400 HTTP status code instead of 201.
• A schedule submitted as a dryrun now returns a 202 HTTP status code instead of 201.
• The note_id property on a schedule collision object will always be present but may be null. It was omitted in some cases.
• The error property on a projected timeslot will always be present but may be null. It was omitted in some cases.

Notes & Caveats:

• The serializers for the scheduling responses are only used for documentation purposes. The scheduling code should probably be refactored to actually depend on those serializers to generate API responses.
• Enums auto-generated by drf-spectacular come with no documentation even though the choices they are generated from may have some. This is the case for the by_weekday attribute of the Schedule instance, as well as the solution_choices attribute of CollisionSerializer and the solutions attribute of the ScheduleCreateUpdateRequestSerializer. I’ve decided to manually patch the generated enums in post-processing as the context information provided by the choice descriptions seems vital to me. It might be a good idea to invest some time into drf-spectacular and send a pull-request upstream so that manual patching can be dropped as one might easily miss this when adding new or changing existing code. See steering/schema.py.

program/migrations/0018_auto_20220322_2113.py

+# Generated by Django 3.2.12 on 2022-03-22 20:13
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("program", "0017_auto_20220302_1711"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="schedule",
+            name="add_business_days_only",
+            field=models.BooleanField(
+                default=False,
+                help_text="Whether to add add_days_no but skipping the weekends. E.g. if weekday is Friday, the date returned will be the next Monday.",  # noqa: E501
+            ),
+        ),
+        migrations.AlterField(
+            model_name="schedule",
+            name="add_days_no",
+            field=models.IntegerField(
+                blank=True,
+                help_text="Add a number of days to the generated dates. This can be useful for repetitions, like 'On the following day'.",  # noqa: E501
+                null=True,
+            ),
+        ),
+        migrations.AlterField(
+            model_name="schedule",
+            name="by_weekday",
+            field=models.IntegerField(
+                choices=[
+                    (0, "Monday"),
+                    (1, "Tuesday"),
+                    (2, "Wednesday"),
+                    (3, "Thursday"),
+                    (4, "Friday"),
+                    (5, "Saturday"),
+                    (6, "Sunday"),
+                ],
+                help_text="Number of the Weekday.",
+            ),
+        ),
+        migrations.AlterField(
+            model_name="schedule",
+            name="default_playlist_id",
+            field=models.IntegerField(
+                blank=True,
+                help_text="A tank ID in case the timeslot's playlist_id is empty.",
+                null=True,
+            ),
+        ),
+        migrations.AlterField(
+            model_name="schedule",
+            name="end_time",
+            field=models.TimeField(help_text="End time of schedule."),
+        ),
+        migrations.AlterField(
+            model_name="schedule",
+            name="first_date",
+            field=models.DateField(help_text="Start date of schedule."),
+        ),
+        migrations.AlterField(
+            model_name="schedule",
+            name="is_repetition",
+            field=models.BooleanField(
+                default=False, help_text="Whether the schedule is a repetition."
+            ),
+        ),
+        migrations.AlterField(
+            model_name="schedule",
+            name="last_date",
+            field=models.DateField(help_text="End date of schedule."),
+        ),
+        migrations.AlterField(
+            model_name="schedule",
+            name="rrule",
+            field=models.ForeignKey(
+                help_text="\nA recurrence rule.\n\n* 1 = once,\n* 2 = daily,\n* 3 = business days,\n* 4 = weekly,\n* 5 = biweekly,\n* 6 = every four weeks,\n* 7 = every even calendar week (ISO 8601),\n* 8 = every odd calendar week (ISO 8601),\n* 9 = every 1st week of month,\n* 10 = every 2nd week of month,\n* 11 = every 3rd week of month,\n* 12 = every 4th week of month,\n* 13 = every 5th week of month\n",  # noqa: E501
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name="schedules",
+                to="program.rrule",
+            ),
+        ),
+        migrations.AlterField(
+            model_name="schedule",
+            name="show",
+            field=models.ForeignKey(
+                help_text="Show the schedule belongs to.",
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name="schedules",
+                to="program.show",
+            ),
+        ),
+        migrations.AlterField(
+            model_name="schedule",
+            name="start_time",
+            field=models.TimeField(help_text="Start time of schedule."),
+        ),
+    ]