Relax AP mention req't that text is full handle

[snarfed]

Right now, to mention an AP user, we require a link to their profile with their full @-@ handle as the text. Is full handle (including instance) really necessary? Does the text matter at all? We should do some tests and see.

See https://fed.brid.gy/docs#mention, #493

[snarfed]

Big question here is whether fediverse servers accept Mention tags based purely on their href pointing to an actor URL or id, or if their name also matters, or even if they need a matching link in the content HTML. Not sure yet, experimenting a bit.

[snarfed]

Did some tests. Mastodon's incoming mention handling is fortunately very relaxed.

• The Mention tag's href can be either the actor id (eg https://indieweb.social/users/snarfed ) or url (eg https://indieweb.social/@snarfed ).
• name isn't required at all.
• The mentioned actor doesn't need to be in cc.
• No HTML link in content is required.

[snarfed]

Done in snarfed/granary@4159f7f