When users submit a no-op edit in Phriction, don't update the document

Summary:
See T1501. When users mash "save", stop them if they didn't change anything.

Also, don't default-fill the "edit notes" field with the previous notes. This is meant to be more like a commit message for your changes.

Test Plan: Edited a document with no changes, got a dialog. Edited a document with a title change only and a description change only, things worked. Edited a document with a previous "Edit notes", got a blank default fill.

Reviewers: alanh, btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1501

Differential Revision: https://secure.phabricator.com/D2978