remove TimeSlotManager and use them as functions

Closes: #180

This is the counterpart to the
requires_express_permission_for_publication field of the License model.

add `needs_author`, `requires_express_permission_for_publication` and `url`

update LicenseSerializer and license fixture

- replace LicenseType with License model, update fixture, serializers, views, and urls

Closes: #167

Closes: #171

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.

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

Replace ModelWithImageFields with foreign key to Image.

Replace ModelWithImageFields with foreign key to Image.

Replace ModelWithImageFields with foreign key to Image.