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.
Cleanup: aligned variables
Small refactoring
Cleanup: removed extra function
Removed unused variable
Renamed variable
Migrated to Underscore templates

Since it's a dependency of Backbone and jQuery-templates' development has
been discontinued, it makes sense to migrate our few client-side templates
to Underscore and thus remove an extra dependency.

Fixes bug 3051.