R1:b799f5671b91

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…