Feedback form (management menu)

It would be nice that people using our system are able to send us feedback. A simple form with a link in the management dropdown menu should do the trick.

My guess is that users will also give feedback quicker when they see they have a simple option to do so.

Todo:
- [x] Write soms tests
- [x] Clean up the javascript (some parts should belong in the assets)
- [x] Don't rely on html5 required field checking 

- [x] Remove everything and write a link