Unit setting and test mode setting Backbone tweaks

Rather than needing to reload the full set of tests from the server-side every time we toggle one of the tests, see the approach I took above, which clears exam mode for any tests currently set (knowing that they will be cleared on the server side when we toggle the current one on or off.

If I were doing it from scratch, I probably wouldn't have `is_exam_mode` as a separate attribute of each test; I would have the API more explicitly match what's happening on the server-side, with a single setting holding which exam is currently active. Maybe we need a TastyPie wrapper on the Settings model (though we'd have to be careful not to expose things like the private key).