This adds a ?branch= filter to the review request API, allowing review

requests to be filtered by branch. This compares directly, and is

case-sensitive.
At the moment, this does not make use of any database indexes, so for

very large installs, this filter may be slow. We are not providing a

database index for branch fields at this time, in order to avoid

impacting the upgrade process for these servers, but if administrators

need to make use of this API and are seeing slowdown, an index on the

branch column can be added manually. We will be indexing this branch in

Review Board 4.0.

Unit tests pass.