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/models.py

@@ -22,6 +22,7 @@ from datetime import datetime, time, timedelta
 
 from dateutil.relativedelta import relativedelta
 from dateutil.rrule import rrule
+from rest_framework.exceptions import ValidationError
 from versatileimagefield.fields import PPOIField, VersatileImageField
 
 from django.contrib.auth.models import User
@@ -673,17 +674,26 @@ class Schedule(models.Model):
         conflicts = Schedule.make_conflicts(sdl, schedule_pk, show_pk)
 
         if schedule.rrule.freq > 0 and schedule.first_date == schedule.last_date:
-            return {"detail": _("Start and until dates mustn't be the same")}
+            raise ValidationError(
+                _("Start and until dates mustn't be the same"),
+                code="no-same-day-start-and-end",
+            )
 
         if schedule.last_date < schedule.first_date:
-            return {"detail": _("Until date mustn't be before start")}
+            raise ValidationError(
+                _("Until date mustn't be before start"),
+                code="no-start-after-end",
+            )
 
         num_conflicts = len(
             [pr for pr in conflicts["projected"] if len(pr["collisions"]) > 0]
         )
 
         if len(solutions) != num_conflicts:
-            return {"detail": _("Numbers of conflicts and solutions don't match.")}
+            raise ValidationError(
+                _("Numbers of conflicts and solutions don't match."),
+                code="one-solution-per-conflict",
+            )
 
         # Projected timeslots to create
         create = []