When subscribed to a new_topic, send endpoints to all apps

[karmanyaahm]

Also needs cleaning and organization, but I'm opening this because I'm very excited to finally have something that works. So we previously discussed that I would do it using just timing data on the app because that would be simpler, but it really was not. Keeping track of the connection state was a whole thing and the worst thing is that it didn't solve the problem of losing the connection on server restart. This implementation only sends out new_endpoint after subscribing to the topic on the ntfy server. Goes along with https://github.com/binwiederhier/ntfy/pull/979