We would like to move some NotifierBot configurations to operate off of a mirror of a repo instead of the original repo. In order to support this I would like to add these two new configuration options in the issue notifier:

1. "originalrepository": Setting this implies that the repo the notifier is operating on is a mirror, and this original repo should be used for any links generated in notifications.
2. "repoonly": Similar to the existing "pronly", this limits what kind of notifications the notifier acts on. This is needed to be able to separate the PR event based notifications from the repo event based notifications. PRs aren't mirrored so those can't be moved to operate from the mirror.