Added some basic resource URL tests, and fixed a bug that these exposed,
that the internal shallow asset locations within deep asset locations
were still discoverable using the BRJS locator methods, leading to
problems.
Removed a test that can no longer work now that we've removed the old
css tag handler.
Created a ResourcesAssetLocation class that can be responsible for
modeling theming, since themes are now a plug-in concern rather than a
model concern.