Backend Improvements

[ngoerlitz]

Some routes are missing the correct checks to determine whether the user is allowed to manipulate this data.
Further, some of the HTTP Methods aren't suitable for the corresponding routes. These should probably be changed, which will require changes in the frontend too!
Lastly, not all routes contain proper input validation yet, potentially causing issues while querying for 'undefined'. The ValidationHelper can be used for this and is widely customizable.