Give administrators selective access to System Agent settings panels

Summary: Ref T4065. Give administrators an "Edit Settings" link from profiles, which allows selective edit of settings panels. Enable Conduit, SSH Keys, and VCS Password.

Test Plan:
  - Used these panels for a bot.
  - Used these panels on my own account.
  - Tried to use these panels for a non-bot account, was denied.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4065

Differential Revision: https://secure.phabricator.com/D8668