make the user experience in ALeA configurable

[Michael Kohlhase]

We now have a couple of intrusive features in ALeA, and they should be make configurable, they are:

• revise bars in the notes (configure whether they are shown)
• when it comes (see #236) the traffic light (configure whether they are shown)
• ...
  There are also other configurations:
• default languges (i.e. a list of the languages a student speaks and understands)
• theme?
  All of these should be part of the user data and should be configurable at https://courses.voll-ki.fau.de/my-profile

I am sure that we will have more configuration possibilities in the future, so it would be good to start having the infrastructure in place.