Add support for selenium UI tests.

UI tests must be in a file matching the pattern `uitest*.py`.
They can be run by using the `selenium` management command.

The tests are not headless, so they require a local X server to be running on the host (Xvfb is not on the Vagrant VM).

We intend to use Sauce Labs for CI, so this shouldn't be an issue for CI builds.
Yes.  As a note, there are some helpers like this (mostly around waiting) on the `SeleniumTestCase` already.  We will want more once we start writing some tests presumably, wouldn't be bad to start off with a place to put them all.