Remove the WebGL checks from the error boundary. Instead, just do not
render the stage if it is not supported.

* 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.

Add Error Boundary (has componentDidCatch method) to handle WebGL errors and display an informative message. 

- implements welcome to the preview modal, and unsupported browser modal.
- will need assets updated in Jan.