Add basic draft support to Phriction

Summary:

• When a user is creating a Phriction document, save a draft as

"phriction:<slug>".

• When a user is editing a Phriction document, save a draft as "<document

phid>:<document version>".

• If a user has an available draft, use that instead of the native content.
• If using a draft, tell the user and give them an option to discard it.
• If a page is updated, your draft is lost (we show new page content

unconditionally) but this should be rare and is the simplest way to resolve this
issue in a realtively consistent way.

Test Plan:

• Recovered drafts for new and edited pages.
• Used "nodraft" to discard drafts.

Reviewers: davidreuss, btrahan, jungejason

Reviewed By: davidreuss

CC: aran, davidreuss

Maniphest Tasks: T769

Differential Revision: https://secure.phabricator.com/D1378