Highlight usernames of the form
@username@twitter.com
in toots, pointing automatically to the corresponding Twitter web page
https://twitter.com/username
For example:
@greatdismal@twitter.com
would be translated into a hyperlink that points to:
https://twitter.com/greatdismal
Tweets don't need this special treatment because they already have an associated URI and the URL for every tweet is accessible via https.