Prevent Harbormaster autobuilds from being stopped, paused or restarted

Summary: Fixes T8657. "Auto" builds are pushed into Harbormaster by an external system (currently, arc) so it does not make sense to stop or resume them: Harbormaster has no way to control the external system.

Test Plan:

• Tried to restart an autobuild, got an error.
• Restarted a normal build.
• Did "Restart All" on a buildable, got restarts on non-autoplans and no restarts on autoplans.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8657

Differential Revision: https://secure.phabricator.com/D13812