R1:afc2f8526bc6

Allow Phriction documents to be deleted

Summary:
- Add a "delete" operation. Delete is just a special edit which removes the
page from indexes and shows a notice that the document has been deleted.
- When a user deletes all the content on a page, treat it as a delete.
- When a conduit call deletes all the content on a page, treat it as a delete.
- Add page status to Conduit.
- Add change type field to history.
- Added a couple of constants to support a future 'move' change, which would
move…