Mastodon extension needs option to choose home instance

[andypiper]

I love the Mastodon extension, super useful - but my account is not on mastodon.social, and it looks like the account login and the top posts are hard-coded to only use that instance.

[zadjii]

It sure does need this. I'd very much welcome a PR for this. Can't really have a fediverse extension if it doesn't actually allow other home servers, right 😅