Allow hyphens in the middle of remote user names (#9345)

Fixes #9309

This only allows hyphens in the middle of a username, much like dots,
although I don't have a compelling reason to do so other than keeping
the changes minimal.

app/models/account.rb

@@ -46,7 +46,7 @@
 #
 
 class Account < ApplicationRecord
-  USERNAME_RE = /[a-z0-9_]+([a-z0-9_\.]+[a-z0-9_]+)?/i
+  USERNAME_RE = /[a-z0-9_]+([a-z0-9_\.-]+[a-z0-9_]+)?/i
   MENTION_RE  = /(?<=^|[^\/[:word:]])@((#{USERNAME_RE})(?:@[a-z0-9\.\-]+[a-z0-9]+)?)/i
 
   include AccountAvatar