models cleanup

http://guides.rubyonrails.org/security.html#regular-expressions

Now it's consistent across layouts where their main menus are.

Also remove now-unused tab_class helper

Simplifies the actions in RepositoriesController

Closes #1452

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

Create just redirects to our specially-formatted #show action

This group is better for features that only deal with one project.