Resolve "Add imprint"

Closes #36

See merge request !4

Resolve "Add privacy policy"

Closes #32

See merge request !3

This reverts commit 9c31d36c.

This reverts commit 4159a966.

- heavily refactored
- changed all async to thenables
- changed all requests from fetch to axios
- axios logs authorization header on login and page reload
- handleError function handles all errors (with the help of some object
  to string parsing function)
- axios logs authorization token on login and on page reload
- only service functions that deliver data really return something
- helpers/requests contains helper functions for requests (as the name
  says)

- from async to .then (or nothing in some cases)
- changed delete and setInvalid (might need more work)

- moved from fetch to axios (and set authorization header at login)
- from async to .then()
- errorhandler helper (in helpers/requests.js)
- overall cleaning