Make Herald transcripts policy-aware

Summary:
Ref T603. Herald transcripts potentially leak a bunch of content (task text, revision/commit content). Don't let users see them if they can't see the actual objects.

This is a little messy but ends up mostly reasonable-ish.

Test Plan:
  - Verified that transcripts for objects I couldn't see no longer appear in the list, and reject access.
  - Verified that transcripts for objects in applications I can't see reject access, albeit less gracefully.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7221