Add mastodon-/fediverse-handle to Profile/Social, akin to the already exisiting Twitter-handle

[Schoeneh]

I've mainly switched from formerly Twitter to the fediverse (mainly mastodon) and would like to add my account to my profile - akin to Twitter.
Automatically adding the rel="me"-attribute would also fix #10102 - but imho the possibility to add this attribute to any link should also exist.