GitLab

There is no incorrect data anymore on the development server.

Closes #17.

Stripe provides an easy to use Checkout modal, which we integrate in our
application.

The course list is completely reworked and now sorts courses by department
and year. Furthermore the individual courses are now nicely formatted.

Additionally, timing overlap betweeen the courses and already selected
courses are now checked.

New validation:

- No timeslot overlap
- user do not only require to use their own nethz, they can also only use this
  nethz if they are members

Additionally tests and README updates.

- Code is now in subdirectory to separate from dockerfile etc.
- script to fill dev container with data now works remotely
- settings accepts more input from environment for better config

The pipeline can now build and deploy!

- Code is now in subdirectory to separate from dockerfile etc.
- script to fill dev container with data now works remotely
- settings accepts more input from environment for better config

Currently amivapi crashes if `user` or `group` are not included in a the
projection for `groupmembership` -- the projection is removed until it is safe
to use it again.

Furthermore added some integration tests that are skipped if tokens are not
provided to not interfere with automated testing.