Move "Delete User" action to user profiles

Summary:
Ref T4065. The existence of two separate edit workflows for users is broadly confusing to administrators.

I want to unify user administration and improve administration of system agent accounts. Particularly, I plan to:

  - Give administrators limited access to profile editing of system agents (e.g., change profile picture).
  - Give administrators limited access to Settings for system agents.
  - Broadly, move all the weird old special editing into standard editing.

Test Plan:
  - Hit all the errors (delete self, no username, wrong username).
  - Deleted a user.
  - Visited page as a non-admin, got 403'd.
  - Viewed old edit UI.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4065

Differential Revision: https://secure.phabricator.com/D8662