Add views and tests for a repository selector.

This change adds views to show a collection of RB.Repository models in HTML
suitable for the new review request UI. This is plumbed through so that the view
will emit a 'selected' event and toggle element classes when the user clicks on
any individual repository in the list.

Testing done:
- Ran unit tests.
- Used this in my new review request UI.

Reviewed at https://reviews.reviewboard.org/r/4353/