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 content from one document to another.

Test Plan:

• Verified deleted pages vanish from the document index (and restoring them

puts them back).

• Verified deleted pages show "This page has been deleted...".
• Created, edited and deleted a document via Conduit.
• Deleted pages via "delete" button.
• Deleted pages via editing content to nothing.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: skrul, aran, btrahan, epriestley

Maniphest Tasks: T680

Differential Revision: 1230