Assignment table dates

[invertedEcho]

• created_at is currently used as start_date, e.g. we misuse it, but should be a normal created_at timestamp, e.g. default now.
• Thus we should create a new column start_date
• This column should store the date in UTC, instead of as right now in UTC+1 / CET
  -> This will require storing the timezone for each user group and adjusting date calculation. This would mean that we finally support not only UTC + 1 users...
• This will probably allow for custom start dates in the future too probably more easily.

[invertedEcho]

This also correlates with #29