Simple Review should disable the approve/reject buttons after they're clicked

[jaredlockhart]

Currently, when a user clicks approve or reject, the button remains enabled while the API call is being made with no visual indication and the buttons are still enabled so there's nothing preventing someone from clicking them multiple times. We should disable the buttons while the API call is being made to prevent users from firing multiple requests.

[leplatrem]

See also https://github.com/Kinto/kinto-admin/issues/1360

[alexcottner]

Closed with PR 3050.