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

@@ -526,10 +526,10 @@ class Schedule(models.Model):
 
                 # Get note
                 try:
-                    note = Note.objects.get(timeslot=c.id).values_list("id", flat=True)
-                    collision["note_id"] = note
+                    note = Note.objects.get(timeslot=c.id)
+                    collision["note_id"] = note.pk
                 except ObjectDoesNotExist:
-                    pass
+                    collision["note_id"] = None
 
                 collisions.append(collision)
 
@@ -607,6 +607,7 @@ class Schedule(models.Model):
 
             projected_entry["collisions"] = collisions
             projected_entry["solution_choices"] = solution_choices
+            projected_entry["error"] = None
             projected.append(projected_entry)
 
         conflicts["projected"] = projected