Also remove now-unused tab_class helper

- Remove compare/_head and just use commits/_head since they're identical
- Add index view; extract the form into a partial

Also moves onchange JS to projects.js.coffee

Seeing `:javascript` all over Views feels like a code smell. This goes a
long way towards reducing the amount of JS in views, but there's still
plenty to be done on that front.

Didn't bother with files in db/, config/, or features/

Use the faster alternative methods in grit to count tags and branches (which I added). This is much faster in all cases, but especially on large repositories the difference should be obvious.