Replace ModelWithImageFields with foreign key to Image.

- 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

this adds the fields `created_at`, `created_by`, `updated_at` and `updated_by`

this adds the fields `created_at`, `created_by`, `updated_at` and `updated_by`

this adds the fields `created_at`, `created_by`, `updated_at` and `updated_by`

this renames the fields
- `ppoi` -> `image_ppoi`
- `width` -> `image_width`
- `height` -> `image_height`
and adds the fields `image_credits` and `image_alt_text`.

this renames the fields
- `ppoi` -> `image_ppoi`
- `width` -> `image_width`
- `height` -> `image_height`
and adds the fields `image_credits` and `image_alt_text`.

this renames the fields
- `ppoi` -> `image_ppoi`
- `width` -> `image_width`
- `height` -> `image_height`
and adds the fields `image_credits` and `image_alt_text`.

this adds the fields `created_at`, `created_by`, `updated_at` and `updated_by`

this adds the fields `created_at`, `created_by`, `updated_at` and `updated_by`