Don't send webmentions to websites that support ActivityPub

[aaronpk]

Scenario:

• My website supports ActivityPub.
• I write a post, send it to my AP followers.
• One of my followers uses Bridgy Fed.
• Someone on Mastodon replies, and their server sends the reply to my website's inbox, and also delivers it to the Bridgy Fed inbox.
• Bridgy Fed fetches the Mastodon post, sees that it's in reply to my post, and sends my website a webmention with a fed.brid.gy proxy URL of the Mastodon post

This causes my website to have received the same post as an ActivityPub reply and as a webmention.

I could update my website to handle de-duping these, but it also seems unnecessary that Bridgy Fed is sending me a webmention when my site already supports ActivityPub.

I think the best middle ground here is to have BF try to find out if a website supports ActivityPub, and not send a webmention in that case.

[snarfed]

Thanks for filing! Original chat discussion here. Interesting point here is that aaronparecki.com isn't a direct Bridgy Fed user, but Bridgy Fed (and Bridgy classic) are promiscuous, ie they send webmentions liberally to any target site they come across, including this one.

The middle ground of checking for AP support via Webfinger sounds totally reasonable here. It'll take a bit of effort, but definitely doable!