Cleanup

I wanted to implement a story and found that an association was missing.
So I pulled on one thread and the whole cloth came with it.
Therefore, I refactored a lot of stuff:

 - link models together
 - no more stupid nested resources
 - remove unused and unmaintained json stuff
 - remove unused controller actions
 - move repetitions in views to partials
 - unify markup of views (can still be improved)

This is likely going to create merge conflicts.
So either merge it now (please!) and maybe rebase on divergent branches.
Merging the other way around will be a huge pain in the ass.
Why is this necessary?