This change adds a new service implementation to handle bug trackers

that don't have their own dedicated service class. The configuration

stores a URL template in settings['url_template'], with %s in place

of the bug ID. This is used to replace the bare Repository.bug_tracker

URL template.
This new service supports bug trackers but not repositories, so it's

configured entirely through a bug tracker configuration and stays out of

the repository form.

Ran unit tests.
Used in conjunction with other changes.