Don't show the delete button on a deleted document

Summary:
This doesn't actually contain any logic to prevent a user from deleting a
document twice, but it makes it significantly harder to do so.

Test Plan:
Went to edit a document, saw the delete button. Clicked it, then went back to
the edit view, and no longer saw the delete button.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3952