backend: `fetcher`: stop after too many failures

When a fetcher has failed too many times, a few things should happen

1. notify senders through pubsub
2. and then shut itself down
3. the notified senders should send an email about the pausing to the users that were subscribed

The UI should show a feed being paused to normal users. And allow admins to restart / alter them. I'll open a separate issue about that once this is done.