Conditionally disable workers activity button

Depends on backend!1350 (merged)
Closes #664 (closed)
Closes #669 (closed)

To avoid fetching the process multiple times, we could inspect the status of the first task. If we are in workers mode, the process has a "pending" activity_state and the first task is complete, then we can patch activity state to "complete" directly in the store. This will require a strong comment.