Allow Also On's with multiple parts

There are a few Also On requests that have two-part handles, most recently the Fediverse, which would cover both Mastodon (a popular Twitter-like) and things like Bookwyrm (a federated Goodreads-like). The two other existing services have a note from 2013 noting that the two-part handle would be tricky, but given the popularity of Mastodon and the existence of the specifically book-focused Bookwyrm, it's worht reconsidering.

This could be implemented with one field for the handle, with some sort of delimiter. For instance, the Fediverse generally uses @xxx@yyy syntax (e.g. I am @c9a@cathode.church) - what would be most natural to Fediverse users is to have one field to put the @xxx@yyy form into, which is then parsed into the URL. This would also make it so it doesn't require changing anything on the database/model side.

I suppose it may require database changes depending on how the Addons configuration information is stored, because it then requires template specifications for both username *and* URL, but hopefully that is a much smaller table if it is in the database, and could even be stored in some sort of combined format (e.g. "@xxx@yyy|https://yyy/@xxx").

As suggested by my musings about implementation, I am a software engineer (currently unemployed, in fact!) and would be glad to pitch in on this feature if that is something that would be helpful.

I'm adding Mastodon in, and it seems that the username should be something like "@username@url" which we'll parse into @username" rel="nofollow" target="_top">https://url/@username