Add a unit test for API root caching.

This adds a unit test to make sure that caching of uri_templates in the API root
takes into account the correct local site.

Testing done:
Ran all tests.
Before the djblets change, the new test failed. After, it succeeds.

Reviewed at http://reviews.reviewboard.org/r/4109/
Add an extra_data JSONField to the user profile

This will be useful for all manner of stuff, including extensions.

Testing done: Ran evolutions.

Reviewed at http://reviews.reviewboard.org/r/4137/
Merge branch 'release-1.7.x'