Classify featherx correctly as using put saver

This should fix a bug where there is no save button for featherx
sites.

Not sure how this worked before, but I think maybe I was a bit lazy
about how the "tw_kind" was implemented/handled for featherx in the
past.

Author: simonbaird

rails/app/models/site.rb

@@ -114,7 +114,7 @@ def core_js_content
 
   def use_put_saver?
     # These always use put saver
-    return true if is_feather? || is_sitelet?
+    return true if is_feather? || is_featherx? || is_sitelet?
 
     # Classic always uses upload saver
     return false if is_classic?