Micro-optimization: only split acct into two Strings (#19901)

* Since `acct` is split by `@` and assigned to `username` and `domain`, we only need to split `acct` into two Strings.

app/models/account_alias.rb

@@ -29,7 +29,7 @@ class AccountAlias < ApplicationRecord
   end
 
   def pretty_acct
-    username, domain = acct.split('@')
+    username, domain = acct.split('@', 2)
     domain.nil? ? username : "#{username}@#{Addressable::IDNA.to_unicode(domain)}"
   end