Replace ModelWithImageFields with foreign key to Image.

Fixes #141

- add `includeVirtual` as GET parameter for `json_playout()`
- refactor `json_playout` to be cleaner

Closes: #120

- remove remaining usage of naive datetime objects
- refactor json_day_schedule and json_playout views
- remove unused methods in TimeslotManager

- Add choices for freq
- Add choices for by_set_pos
- Add unique_together

Fixes: #133

Only return the next repetition timeslot if there is one

Only return the next repetition timeslot if there is one

Fixes #131