fix dragging new attributes among groups

Summary: the issue wasn't dragging between groups, but that new models weren't being saved immediately, so the drag would fail. Previously saved models could be dragged between groups without an issue. Not sure if the logic to not save models immediately was necessary... Fixes T895

Test Plan: try creating new attributes on forms then dragging them to new form groups, check that they're saved. Also check that this is possible with existing attributes.

Reviewers: rjmackay, dkobia, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T895

Differential Revision: https://phabricator.ushahidi.com/D425