Allow Phriction document edits to be saved as drafts

Summary:
Depends on D19661. Ref T13077. See PHI840.

When a user edits a page normally, add a "Save as Draft" button. Much of this change is around making that button render and behave properly: it needs to be an <input type="submit" ...> so browsers submit it and we can figure out which button the user clicked.

Then there are a few minor rules:

• If you're editing a page which is already a draft, we only give you "Save as Draft". This makes edits to update/revise a draft more natural.
• Highlight "Publish" if it's a likely action that you might want to take.

Internally, there are two types of edits. Both types create a new version with the new content. However:

• A "content" edit sets the version shown on the live page to the newly-created version.
• A "draft" edit does not update the version shown on the live page.

Test Plan: Edited a published document, edited the draft. Published documents. Reverted documents.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19662