Username mentions regex may be too permissive
As an extension of #239 (closed)...
https://mastodon.cloud/@wilw/100635802111245669 does not parse as a link because it uses an unknown TLD (.cloud). That in itself is not so bad, but it results in a mention being parsed inside of it (
This may just be working-as-intended but perhaps username mentions should only work when surrounded by whitespace.