Remove WebAppFixture

The fixture pattern has been less useful than I anticipated.  In particular, I've often needed to get information *back* from the fixture: for instance, getting the location from an XBlock created using the Studio API.  Since fixtures are installed in the `setup()` method of the test case, there isn't an easy way to get this information.  And once I remove fixtures from `WebAppTest`, they're basically an abstract base class with one method, which is useless :)

For this reason, I want to remove fixtures from bok-choy, but continue using the design pattern in tests.

@jzoldak