R1:580790cd6e35

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…