This change adds several new properties to the bug-tracker service implementations: - bug_tracker_label` is used as the default field label for trackers on that service (e.g. a GitHub repo will show "GitHub Issues:" instead of "Bugs:") - `supports_bug_info` is a flag that will allow us to more efficiently check if a hosting service includes the `get_bug_info()` method. - `supports_bug_search` is a new flag to indicate whether a bug tracker backend supports interactive searching for bugs. - `bugs_in_repo` is a flag to indicate that a service's tracker in bound to its repository (e.g. GitHub Issues). For services where this is the case, the tracker is added individually for each repo instead of having a central config that can link to multiple repositories. Testing Done: Ran unit tests.