Replace ModelWithImageFields with foreign key to Image.

- remove ThumbnailsMixin and thumbnails fields,
- remove image_* fields and theis usage,
- use utils.delete_links function

Replace ModelWithImageFields with foreign key to Image.

- remove ThumbnailsMixin and thumbnails fields,
- remove image_* fields and theis usage,
- introduce utils.delete_links function

Replace ModelWithImageFields with foreign key to Image.

Closes #142

- replace `run.sh init-db` script with `initialize` Makefile target,
- run `migrate` and `loadfixtures` as prerequisites to `initialize` target,
- add `run.dev`, `run.debug` and `run.prod` targets,
- add `migrate` as prerequisite for run targets.

Update Dockerfile accordingly

Closes #127

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