Fix Local Site and validation issues with post-commit hooks.

The post-commit hook support in hosting services had a few issues that
could lead to unintentionally closing the wrong review requests on
other repositories. They also weren't compatible with Local Sites.

The URL for the hook identified the repository, and the URL also
contained the hosting service ID, but we wouldn't use either to properly
validate the repositories. That meant that if a hook was set up to point
to repository 1 on GitHub, and then had a commit referencing a review
request on repository 2 on Bitbucket, things would break.

We now capture the hosting service ID and pass it, the local site name,
and the repository ID to the function that handles closing review
requests. That function will then use that information to narrow down
the list of repositories being processed, in order to avoid operating on
some other review request. The function has also been rewritten to
minimize the number of SQL queries needed.

Local Sites were broken because nothing bothered to check for them. The
URLs were added to the root urlpatterns, and not to
localsite_urlpatterns. There was some code that did something weird with
trying to get a Local Site from a resolved URL, but that didn't really
do anything. Now everything is wired up to allow for hooks on Local
Sites and to restrict operations to review requests on the Local Site.

There's a few other changes here and there to help consistency and to
minimize code. For instance, we had different error responses for
invalid payloads, and they weren't really correct. We just use HTTP 400
now.

Unit tests were added for the three services that support post-commit
hooks. They test hooks with and without Local Sites, and test invalid
repository IDs, Local Site names, and service IDs.

The unit test for testing service registration was also split up into
several tests, since it was testing several things.

Reviewed at https://reviews.reviewboard.org/r/6303/