Update to use Djblets's new extension imports.

This updates the imports to pull from the new locations in the
djblets.extensions module. It leaves the Review Board extension modules
as they were.
Update to the latest version of django-pipeline.

Now that django-pipeline has an explicit setting for enabling/disabling
media pipelining, and now that we're using Django 1.5, we can make use
of it again. This upgrades us to the latest release of pipeline.
Fix switching to Custom repositories and missing hosting services.

When switching the hosting service of a repository from a service to
Custom, the repository would stay on the hosting service. This was due
to a repository still being associated with a HostingServiceAccount. We
now set that to None when switching.

There was also an issue loading the administration page for repositories
when a hosting service was missing (due to an extension not being
enabled). If a repository had a hosting service account, it was assumed
that it could load that account, and that would fail, breaking the
listing of repositories. We're now a little more careful in this
particular code path.
Merge branch 'release-1.7.x'