R1:f5e1e3377c71

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…