Rewrote merge_requests.js as coffeescript and updated code

Rewrote `merge_request.js` as coffeescript.

Updated the naming of classes to be less specific and more hierarchical e.g.:
`.mr_nav_tabs` becomes `.merge-request .nav-tabs` where `.nav-tabs` is already a global element for navigation tabs. 

Same for `.merge-request-diffs => .merge-request .diffs`, `.merge-request-notes => .merge-request .notes` etcetera.

And this way actually all elements of the merge request page are wrapped inside a div with class `.merge-request`