Allow configuration of default document policies for Legalpad

Summary: Ref T3116. User request / generally modern feature.

Test Plan: Set defaults to whacky projects and created a new document; it defaulted appropriately.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, allan.laal

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D8110