Fix IDN mentions not being processed, IDN domains not being rendered (#12715)

This changes the REST API to return unicode domains in the `acct`
attribute instead of punycode, and to render unicode instead of
punycode on public HTML pages as well.

Fix #7812, fix #12246

app/helpers/accounts_helper.rb

@@ -13,7 +13,7 @@ module AccountsHelper
     if account.local?
       "@#{account.acct}@#{Rails.configuration.x.local_domain}"
     else
-      "@#{account.acct}"
+      "@#{account.pretty_acct}"
     end
   end