Add support for self-hosted hosting services.

Now, hosting services an be marked as "self-hosted," which will cause a
"Domain" field to show up for the service above the account entry. Many
of the functions now take a domain.

Accounts now take a domain as well, and we filter registered accounts by
the domain.

Bug trackers have the same sort of setup as well.

GitHub's had a few more changes added to it for cleanup to use the
hosting URL and to not duplicate so much code.

Unit tests were added for all of this.

Unit tests pass.

Tested this extensively with another change that makes use of this.