A service can support bug trackers without supporting repositories or a legacy bug tracker URL template, configuring itself entirely through a bug tracker configuration. Three places assumed otherwise: * `RepositoryForm` listed every bug-tracker-capable service in its bug tracker type field. A service with no `bug_tracker_field` template and no plans has nothing for that form to configure, and building its (nonexistent) form logged an error. These are now skipped. * `get_bug_tracker_requires_username()` assumed `get_field()` returned a string. `bug_tracker_field` defaults to `None`, so guard against it. * The hosting service API resource synthesized a default plan for every service without plans, which requires a repository form. Services without one now serialize as an empty set of plans. Testing Done: - Ran unit tests. - Used in conjunction with other changes.