Implement PhrictionDocumentQuery

Summary:
Companion for D5284. Move all the query logic to a policy-aware query class. In particular:

• Currently, anyone can view and edit a project's wiki documents. For callsites using this query class, you must be able to view or edit the project to view or edit its documents.
• There's some very sketchy logic with the content/document joins. This cleans that up.
• This cleans up loading projects by moving it inside the query. We need to do this anyway to perform policy checks.

Test Plan: Viewed active/all/updated. Set page size to 2, verified pager works.

Reviewers: AnhNhan, chad

Reviewed By: AnhNhan

CC: aran

Differential Revision: https://secure.phabricator.com/D5285