Schedule updates of repository index files

Once #755 (closed) is done, we'll need a way to schedule updates of repository index files when the contents of a debian:suite collection change. In the past we've talked about some kind of general publish/subscribe mechanism, but that's probably overkill for this use case.

Perhaps we can do something like scheduling a Celery task in such a way that the index files are only updated once if the collection is changed multiple times between generation runs.