Closes #203

This reverts commit ad526281.

Closes: #187

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