Mastodon should send webmentions to linked sites.

Webmentions are a way to notify sites that they have been linked on a blog or social media. While #1384 is focused on Mastodon processing webmentions from outside, I think that it would be easier and more useful in the short term to have Mastodon attempt to send a webmention to sites that are linked in a toot, with the permalink of the toot.

https://webmention.net/draft/

Mastodon already has code to fetch the site (for generating a card), so parsing the result for a webmention endpoint should be a short step. Sending webmentions documents the process:

The sender MUST fetch the target URL (and follow redirects [FETCH]) and check for an HTTP Link header [RFC5988] with a rel value of "webmention". (Alternate endpoint discovery methods discussed here too)

The sender MUST post x-www-form-urlencoded [HTML5] source and target parameters to the Webmention endpoint, where source is the URL of the sender's page containing a link, and target is the URL of the page being linked to.

• I searched or browsed the repo’s other issues to ensure this is not a duplicate.