Add filtering for the repository selector.

This change adds a search box for the repository list, which should help for
servers that have a ton of repos connected.

Testing done:
* Tested the animation back and forth to make sure everything was silky smooth
  and drew correctly.
* Added a couple repositories with different names. Checked that searches
  worked as expected.
* Ran jshint.

Reviewed at https://reviews.reviewboard.org/r/4526/