Add draft support to ApplicationTransactions

Summary:
When previewing, save drafts. When loading objects, restore drafts if they are available.

Depends on: D665

Test Plan:

• Viewed a Mock.
• Typed text into the comment box.
• Reloaded the page.
• Text still there.
• Hit submit, got my comment.
• Reloaded the page.
• Draft correctly deleted.
• Repeated for Macros.

Reviewers: btrahan, chad, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2104

Differential Revision: https://secure.phabricator.com/D4252