Add a field for Mastodon profile

[ligurio]

Probably Twitter is less relevant than earlier and field for Mastodon profile could be useful.

This issue requests an additional field for Mastodon.

[hishamhm]

I have close-to-zero frontend skills but as someone who's also made the move from Twitter to Mastodon/Fediverse, I'd love to see it happen :)

[hishamhm]

In case anyone looks into this: I just tried changing my "Twitter account" to @hisham_hm@mastodon.social, and it failed with the following:

    profile: twitter: expected valid text then text then type "string" length_type range from 1 to 20 then twitter username, or empty

so, if this was to change into a "Twitter/Fediverse account" field (probably the easiest approach?), it would require some migration to increase the max length and relax the formatting rules to accept the two at-signs...

[leafo]

The rendering code will also have to be updated to make sure the correct URL is put in to the page, and it has the rel=me attribute.

I don't see any issues with expanding the syntax, so that @hello defaults to twitter and @hello@world can be used to specifically specify a domain