Shrink sensor-model view
rm excessively complex PlaybackController changes

This reverts src/lab/common/controllers/playback-controller.js to cf29dca7b. See https://github.com/concord-consortium/lab/blob/cf29dca7b8aeb09e61b116ee8b0a6ff93159ebe2/src/lab/common/controllers/playback-controller.js

The changes in the intervening time had to do with the removal of the 'model' global variable. The required changes can be made much more simply.
PlaybackController: handle non-global model simply