Also, fix detection of IE so it works. Apparently bowser.name !== 'IE' on IE with bowser. bowser.msie works though.

bowser correctly detects Chrome Headless as Chrome, is already used by scratchr2, and is more actively maintained than platform.

* Added object.assign polyfill

* Fix IE load failure

Don’t call `(window.AudioContext || window.webkitAudioContext)()` if platform is IE

Catch any other errors, and display Browser unsupported in error-boundary.

If for some reason there are no other errors (unlikely), preview-modal will still check for IE and show the unsupported modal.

Polyfilling Object.assign is enough to get through load and show the modal for IE. We can always switch to `babel-polyfill` if there are others needed later, but for now this is smaller.