[WIP] aspect membership dropdown Backbone.js rework

*PREVIEW* - not fully functional right now - *DO NOT PULL YET*

re-do the aspect membership dropdown - backbone style
... to get rid of even more of the legacy javascript code

(route remains as is, but more REST-ful, like described here: http://ngauthier.com/2010/11/restful-many-to-many-relationships-in-rails.html)

will solve #3829