This adds the initial admin forms for bug tracker configurations. This

works similar to the existing repository form: the main BugTrackerForm

includes settings for the name, service, account (where applicable),

review request scoping, and acting-user access conditions.
Service-specific subforms get added, based on the selected service.
The new admin definition sets up the form, and overrides get_queryset

so that it never lists the hidden sentinel row.

Ran unit tests.