Re-do feed discovery on subscription renewal
When a PuSH subscription periodically has to be renewed, it should execute discovery on the profile first in case it changed. As it is right now, if that URL changes, it will never be able to update again so long as a single person is still following the remote resource.
This can happen in the following circumstances:
- when following a generic PuSH-enabled feed rather than a GS user
- when mastodon changed from using superfeedr to its own PuSH server
Regardless, the way it works now is incorrect behavior as defined by pubsubhubbub since it must be assumed that feeds will periodically change hubs.
I'll try to fix this myself and submit a merge request, I just didn't want to forget about it.