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.

Loaded the list of repositories when it was previously in a broken state.

It loaded fine.
Switched a Bitbucket repository to Custom, and saw it stick, instead of going

back.