This adds TestCase.siteconfig_settings, a context manager that

overrides the site configuration settings for a test or part of a test.
The base implementation will store new settings in siteconfig and then

revert them after the context manager is finished. It does not apply any

settings to Django by default, though, leaving this up to a more

specific implementation.
This is largely pulled from a utility method in Review Board.

Made use of this in upcoming changes. Verified the new siteconfig settings

and that they were reverted after the change.