Filter repositories based on a user's access.

This provides a Repository.objects.accessible() query function that allows
for filtering based on a user's access to the repository. It can also optionally
show hidden (but accessible) repositories if visible_only is set to False.

All call sites have been updated to use the new accessible() function and to
check if a repository is accessible when accessing it directly. This includes the
New Review Request page.

New unit tests pass.

Verified that the repos were limited based on whether I had access (when they were private).