Add a "Can Create Documents" permission for Legalpad

Summary: Ref T3116. Installs might reasonably want to restrict creation of these documents to actual lawyers or something.

Test Plan: Adjusted policy, tried to create document, set it back, created a document.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D9778