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

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.