This change adds an implementation of get_bug_info_uncached for GitHub

repositories. This is relatively straightforward (hurray for bug trackers which

give us sane results in json!). The one change I had to make was to make it so

Repository.bug_tracker_service returns an instance rather than a class, and

handles bug_tracker_use_hosting correctly.

Added the Review Bot repository to my server, and told it to use the hosting

service's bug tracker. Saw that links to the bugs worked, and that hovering

over bug IDs showed the infobox with the correct data.