Save stacked actions in drafts, not just comments

Summary:
Ref T9132. Fixes T4580. Thhat might actually have been fixed a while ago or something since it describes a buggy/bad interaction which doesn't reproduce for me at HEAD.

This saves and restores all the stacked actions (subscribers, projects, etc) so that you don't lose anything if you close a window by accident.

Test Plan:
Added a bunch of actions in various states, reloaded the page, draft stuck around.

Submitted form, actions didn't stick around anymore.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4580, T9132

Differential Revision: https://secure.phabricator.com/D14675