R1:1729e7b46792

Improve UI for "wait" and "answered" MFA challenges

Summary:
Depends on D19906. Ref T13222. This isn't going to win any design awards, but make the "wait" and "answered" elements a little more clear.

Ideally, the icon parts could be animated Google Authenticator-style timers (but I think we'd need to draw them in a `<canvas />` unless there's some clever trick that I don't know) or maybe we could just have the background be like a "water level" that empties out. Not sure I'm going to actually write the…