Improve rendering of new files in the diff viewer.

New files no longer have the blank line number and code columns for the
left-hand side of the diff. It just has the new code's columns.
Update diffviewer unit tests to pass newly required data.

One of the unit tests broke with my new change for improving the display
of new files in the diff viewer. It wasn't provided all the expected
data. It's been updated to pass it.
Use raw ID fields for the ReviewRequest.depends_on and repository fields.

ReviewRequest's depends_on and repository fields can cause massive
queries in the admin UI. They're now set to use raw ID fields instead of
showing lists of possible items.
Add generated CSS files to the gitignore list.
Fix the color of the border between navbar and dashboard.

The border color was black, which stands out in the modern dashboard.
It's now using the same color as the dashboard border, blending in
nicer.
Add views and tests for a branches selector.

This change adds views to show a RepositoryBranchesCollection using a <select>
tag. This is used in the new post-commit review request creator.

Testing done:
- Ran unit tests.
- Used this in my post-commit UI.

Reviewed at http://reviews.reviewboard.org/r/4321/