Add free-form social media fields, or add more variety

[SaschaMann]

A lot of people have social accounts that aren't on Twitter, LinkedIn etc they might want to link. It would be nice to have either more options or free form fields that one could add in those places.

For Mastodon, I'd additionally like it if the link was set with a “rel=me” so that it shows up as verified when I link to my Exercism account from Mastodon.