Allow PubSub Hub url to be configured

[TheSilkky]

This will let users running Piped inaccessible from the public internet to self host their own WebSub / PubSubHubbub 0.4 compatible server to update their subscriptions feed.

Should help with TeamPiped/Piped#707 and TeamPiped/Piped#1130

[TheSilkky]

Personally, I've been using this implementation: tystuyfzand/websub-server

[FireMasterK]

Thank you! But how would you host a websub server? Is there any docker image perhaps for the repository you linked? If possible, I would link a local server directly for new installations.

[TheSilkky]

I made a mistake opening this, I initially thought this worked because I wrongly assumed my feed had updated. I've since realized that this won't work because the feed still has to notify the hub to fetch new data via a POST request.

I did some experimenting with using timers instead of a request from the feed to initiate fetching, but as it turns out YouTube doesn't actually publish the data at all and it's only accessible when using their hub.

Ultimately, my solution is to connect a cheap vps to my private network, use it to expose the /webhooks/pubsub path on it's own public address and then configure the PUBSUB_URL option to that address.

Sorry for wasting your time with this, I think I just got a little excited lol