Added a decorator to get the resources for a translation project view

This will retrieve directory and store objects when necessary and the
request object will be populated with them.
One less query for subdirectories
Source language is not used

...and it actually needs an extra query, which we can avoid.
Unified single- and multi-file URLs

Fixes bug 3015
Use get_absolute_url instead
Let get_absolute_url point to the overview page
Simplified VCS commit/update URLs and views
This needs to be fixed sometime, let's not forget about it
Renamed decorator for clarity
Moved permission checking into its own function
Removed unused imports
Added decorator to set the resource context in XHR calls
Use a single endpoint for retrieving failing checks via XHR
Removed unused limit parameter
Unified XHR editor URLs
Moved third-party libraries into a separate directory
Added Backbone and Underscore
Some URLs changed, adjust captcha middleware
Refactored _build_units_list() for further reuse
Decouple editor from translation projects

This is the biggest piece chunk of work to decouple the editor from
translation projects (bug 3013). There are still fixes and more cleanups
pending, but this should give a good base to start working on
cross-language/-project scenarios such as bug 2719.

The indexing part has been commented out for now, and that should be
addressed as part of bug 3028.