Support abstract/interface event classes

A common use case for this would be in listening for permission granted events and possibly blocking them. Currently you have to specify global, repository and project level event classes:

GlobalPermissionGrantRequestedEvent, ProjectPermissionGrantRequestedEvent, RepositoryPermissionGrantRequestedEvent

Allowing abstract event classes to be listed would allow you to just specify one class to listen for all the permission grant events: PermissionGrantRequestedEvent

This may have performance implications if your listening for an abstract event that's fired many times, but we could have an in product warning about this when one is selected, or something in the documentation.