Make the update people page atomic

[adehnert]

When adding and removing people, perhaps updates should be atomic --- either everyone gets added/removed, or nobody does. The advantage of this is that it's harder to lock yourself out by removing yourself and simultaneously typoing the person you're adding. The disadvantage is that if you're doing a bulk edit, one mistake can force you to go back and recheck all the boxes. I'm not sure which is better.

To some degree, fixing ASA-#71 would be an alternative solution to this issue.