Disable start button on task submission

[wpbonelli]

When the start button is clicked on the task submission view we should disable it until a response arrives from the backend. This prevents accidental duplicates.

I thought we already did this? Must have regressed somewhere along the line...

(It would be nice if the response were quick enough to make dupes impossible but good to take precautions- can probably optimize the endpoint to shave some time but that's a separate issue)