Edit proposal and badges in one overlay

This adds a configruation option `adhocracy.proposal.split_badge_edit`. It defaults to `True` which produces the same behavior as before. If set to `False` however the proposal "edit" and "edit badges" forms will be combinend into one, the now obsolete button "edit badges" in proposal view is not displayed and the small "edit badges" button in the proposal index changes to "edit".

Putting the proposal edit form into an overlay is done using iframe overlays and #579. This should therefore be merged after #579. Also note that this PR contains *additional commits* as it is rebase against #579.

I removed the old knockout.js code which provided the edit badge form before. While the new code is much more flexible it is also slower and a bit rougher on the edges. I did not do a lot of work verifying that the code is not actually used anywhere else so please tell me if I missed something here.

I also refactored the badge edit html.