Remove ReleephUserView

Summary:
Fixes T3738. Facebook uses this to provide a couple of integrations (push karma, is user an intern?), but the mechanism is both very complex and not very general.

Instead, these features are better implemented in Hovercards or via CustomField. We'll help Facebook integrate things when the time comes, but per discussion in T3738 none of this is critical or especially complicated.

Test Plan:
  - Grepped for all callsites.
  - Viewed a request and verified that author/requestor populated and rendered correctly.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3738

Differential Revision: https://secure.phabricator.com/D8631