Fix wrongful matching of last period in extended usernames

Fix anchor tags in some wikipedia URLs being matches as a hashtag

app/models/account.rb

@@ -4,7 +4,7 @@ class Account < ApplicationRecord
   include Targetable
   include PgSearch
 
-  MENTION_RE = /(?:^|[^\/\w])@([a-z0-9_]+(?:@[a-z0-9\.\-]+)?)/i
+  MENTION_RE = /(?:^|[^\/\w])@([a-z0-9_]+(?:@[a-z0-9\.\-]+[a-z0-9]+)?)/i
   IMAGE_MIME_TYPES = ['image/jpeg', 'image/png', 'image/gif'].freeze
 
   # Local users