* 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.

Temporary message when the GUI has an unhandled error

Fix firefox display and functionality issues with sound edit

Reorder motion category

temporary patch for #1205.

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

Update scratch-l10n to the latest version 

* Update libraries re: feedback. Resolves GH-1182 GH-1187 GH-1188 GH-1189 GH-1184

* Rename astronaut in space theme

center/size stage-header controls in the same way as stage in fullscreen

Fix duplicate birthday sound clips

Set up S3 deployment

Use new looks reporters with menus for costume name and number.

Improve performance and cosmetics

Remove sprite-specific text from sprite/sound/etc deletion warning

- reduced the load time of the preview modal assets
- changed cursor to ‘pointer’ for modal buttons

* Change default variable name to 'my variable'. 

* Update integration test to look for 'my variable'. Re GH-1162

* Resolve issue with space in integration test

Update opcode label for monitors.

Update scratch paint version number manually

Minor revisions to new sprites and costumes

* Implement paint-75: Add blank costume button

* add backdrop icon, use default name

thanks @fsih !

Update preview illustration

Replace the placeholder illustration.

Update to new XY icons

Add next/previous backdrop options in backdrop block

Update chromedriver to the latest version 

Add new sprites