Use it with `HostSerializer`, `NoteSerializer` and `ShowSerializer`

- reorder explicitily named fields,
- define `{created,updated}_{at,by}` as read-only-fields

- `HostLinkSerializer` is a `serializers.ModelSerializer` again.

- fixed image field names,
- removed fields that are no longer used

- `HostLinkSerializer` is a `serializers.ModelSerializer` again.

- fixed image field names,
- removed fields that are no longer used

The following attribute names were marked as deprecated in the schedule
request and have been replaced by their new names:

  dstart → first_date
  until → last_date
  tstart → start_time
  tend → end_time
  byweekday → by_weekday

Apart from that this commit fixes a few documentation errors.

When making the serialized fields explicit in
0ee3a655 we forgot to add the id field
that is used in various places.