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/serializers/rest/account_serializer.rb

@@ -24,6 +24,10 @@ class REST::AccountSerializer < ActiveModel::Serializer
     object.id.to_s
   end
 
+  def acct
+    object.pretty_acct
+  end
+
   def note
     Formatter.instance.simplified_format(object)
   end