This updates WebHookTargetForm to use the new mixin for working with

LocalSites. This allows to be optionally bound to a LocalSite and to

automatically validate against the site.
The form code itself is now a bit simpler, as it doesn't have to perform

its own validation of any specified repositories. It will also allow

future work on the form without worry about injecting incompatible

LocalSite-bound data.
New unit tests have been added for the form, since we didn't have any

before. These largely mirror the tests we have for

LocalSiteAwareModelFormMixin, but with data specific to this form.

This helps to ensure that we don't somehow make the form less secure as

changes are made to the mixin or other code.

Unit tests pass.