Unfatal rendering of repository policy transactions

Summary: Fixes T4919. There's some special casing in Diffusion for CAN_PUSH right now, just accommodate that until things get more general.

Test Plan: Viewed a repository edit screen with a custom policy transaction. Clicked the link to view it.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4919

Differential Revision: https://secure.phabricator.com/D8898