Wrong mix of timezone aware and timezone naive datetime objects

Close #116

Closes: aura#164

Since this may lead to errors while initialazing the container.

Closes: #163

Closes #139

- 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

- Run as aura with AURA_UID
- Use WORKDIR /app
- Install Poetry in /opt/poetry

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