Policy Transactions - add a details view for custom policy

Summary: 'cuz those can be complicated. Fixes T4738. I needed to do a fair amount of heavy lifting to get the policy stuff rendering correctly. For now, I made this end point very one purpose and tried to make that clear.

Test Plan: looked at some custom policies. see screenshots.

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4738

Differential Revision: https://secure.phabricator.com/D8890