Support WebSub

[pstuifzand]

Not only fetch, but also subscribe to WebSub enabled feeds (and webpages). This way we can update the items inside microsub server.

[pstuifzand]

This should add resubscribe for feeds and hubs. I guess we should remember the expiration time for the subscription and resubscribe before the subscription ends.

[pstuifzand]

Resubscription was added, now I need to find some feeds with WebSub and see if those work.

(Originally published at: https://p83.nl/p/784)