Custom Policy Editor

Summary:
Ref T603. This isn't remotely usable yet, but I wanted to get any feedback before I build it out anymore.

I think this is a reasonable interface for defining custom policies? It's basically similar to Herald, although it's a bit simpler.

I imagine users will rarely interact with this, but this will service the high end of policy complexity (and allow the definition of things like "is member of LDAP group" or whatever).

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran, asherkin

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7217