Four tests in `test_notes.py` still broken for now

Closes: #155

- Add `pytest-factoryboy` dependency
- Add pytest options
- Add factories for User, Host, Image, FundingCategory, Type, Show, RRule, Schedule and Timeslot
- Add pytest fixtures
- Add tests for Schedules, Shows, Timeslots ma Users

The seralizers for Host, Note and Show require the request in the
context, but only the `APINoteViewSet` calls `get_serializer_context`.
The other two `APIHostViewSet` and `ShowSerializer` ingnore it, and the
request needs to be added manually.

- The user or username is no longer "injected" in the view sets
- The user and username are obtained in the serializers from requests

1. remove the fields
2. re-add the fields as not-nullable.

1. remove the fields
2. re-add the fields as not-nullable.

This reverts commit 1546a2c2.

This reverts commit 18dd866f.

Make Category.subtitle a CharField

- Added `created_{at,by}` and `updated_{at,by}` to `Host`, `Show`, `Note` and `UserProfile`.
- Sorted all the fields alphabetically (still not 100% sure it is more readable)

- Replace ImageField with ForeignKey reference to Image
- Update `program/show.json` fixture
- Update serializer to refer to Image and allow null

The serializer would return an invalid format for the image.ppoi field
causing errors on save through the API even if the API consumer didn’t
modify the ppoi field value.

Apart from that the current implementation caused an internal server
error instead of rejecting invalid formats on save.

Any frontend will need the dimensions of the supplied thumbnails
to select the appropriate thumbnail in different rendering contexts.

Apart from that the thumbnail name is not that helpful, as we need
the full thumbnail URL to be able to display it.

Wrong mix of timezone aware and timezone naive datetime objects

Close #116

This is mainly so we can start phasing out nested routes.
With nested routes objects were filtered based on route parameters.
Therefore we need something similar in the form of query parameters.