Restrict (Partial)UpdateTask to process admins

Closes #1540 (closed)

PartialUpdateTask is used by the frontend to allow stopping a single task and restarting it. The frontend only displays this to instance admins, but this has always been allowed to process admins. I chose to keep the latter, but with all this permissions cleanup, it will be easier to switch to instance admins later (or display the buttons properly in the frontend).