Support XFN identity (Mastodon verification) links in profiles

Description

Very loosely related to #11580

Currently, without any additional plugins, there is a field that can be used by users to link to their social media account - the website field. However, it's just a singular field (so one account only), and it currently won't allow for profile verification on Mastodon (and other social media using XFN rel="me" links for verification).

So since NodeBB is working towards supporting AP, I believe supporting the use of fediverse networks in a simple way would be a good idea :) Additionally, it'd open up the way for supporting verification back.

In total I have three proposals here:

• add me attribute to user links
  • currently they have rel="nofollow noopener noreferrer" - adding me there shouldn't affect SEO AFAIK
    • side note: noopener there should be redundant since noreferrer sets it automatically and is actually older than noopener so it doesn't even provide any legacy browser support. noopener is supposed to be used specifically when you don't want to set Windows.opener while still passing the Referer header
    • ref: NodeBB/nodebb-theme-harmony#24
• support multiple profile links
  • I'd say ideally it should also support some kind of metadata (that is, some name for the link), similar to how Mastodon does this.
  • I'm not sure how should this be done with regards to template compatibility though. Passing the first link the way it's currently done (which actually includes backend-generated websiteName, so that's done :) and the rest as a new attribute would work, but seems really ugly. I guess it could be then unified in NodeBB 4.0?
• support link verification
  • would require NodeBB to check the website entered to look for a link with rel attribute that includes me

Community forum reference

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