Don't show deleted pages from the 'All Documents' panel.

Summary:
If a user is asking for a list of documents stored in Phriction, it's pretty
safe to assume that they mean documents that actually exist.

Test Plan:
Made a document, saw it listed in /phriction/list/all. Deleted it, and no longer
saw it.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3951