Option to federate posts from feeds automatically

[snarfed]

Right now, to federate posts into the fediverse, users have to trigger them automatically by sending a webmention per post. This is useful for people who only want to federate some of their posts, but it can be inconvenient for others who want to federate all of them.

We could add support for federating all posts in a feed (h-feed, Atom, RSS) automatically. Two catches:

• We'd need to start subscribing to and regularly fetching feeds, which is new load. Could use https://superfeedr.com/ to avoid that load, and/or WebSub for sites that support it.
• I'd probably want this to be opt in, or at least opt out, and I don't have a UX way for sites to choose that yet.

(Requested by @jamietanna et al)

[snarfed]

For people who just want posts to show up in Mastodon, and don't need Bridgy Fed's richer bidirectional interaction support, there are lots of existing RSS/Atom tools:
https://www.google.com/search?q=rss+to+mastodon
https://www.google.com/search?q=atom+to+mastodon

[snarfed]

Tentatively closing this. Lots of other tools that do this, and hopefully the difference will be clearer when I remove the Atom mention from the docs entirely. #283, #295