Add basic firefish support

[Bo-Duke]

Firefish is close to Mastodon in features, meaning that users from both services are likely following each other without even knowing.

Basic support for firefish should be added, depending on what the API provides, a minimum would be listing firefish instances at the bottom of the page (currently they're in the error card, with down instances) and showing a warning message when the provided username is on a firefish instance.