the waveshaper node is implemented incorrectly on Mac Safari (https://bugs.webkit.org/show_bug.cgi?id=129333) and causes extreme distortion of the sound

This reverts commit 45cb96f2.

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.

It claims to support other sample rates, as the spec requires, but it
does not.

* 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.