Count all URLs in text as 23 characters flat, do not count domain part of usernames (#4427)

* Count all URLs in text as 23 characters flat, do not count domain part of usernames

* Add new status text counting logic to web UI

app/models/account.rb

@@ -44,7 +44,7 @@
 #
 
 class Account < ApplicationRecord
-  MENTION_RE = /(?:^|[^\/[:word:]])@([a-z0-9_]+(?:@[a-z0-9\.\-]+[a-z0-9]+)?)/i
+  MENTION_RE = /(?:^|[^\/[:word:]])@(([a-z0-9_]+)(?:@[a-z0-9\.\-]+[a-z0-9]+)?)/i
 
   include AccountAvatar
   include AccountFinderConcern