Closes #152

To be honest I'm not sure this is the right way to do it.

Closes: #155

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