Since we will be adding more interactivity, the active state will
eventually pop the menu open, so I'm not worried about accessibility of
this right now.

This is a long standing issue with how iOS deals with scrollable
elements within a fixed area. This was a helpful resource
https://davidwalsh.name/scroll-iframes-ios

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

Additionally render the canvas at a multiplier of the device pixel
ratio, so we get hidpi images for supported screens. The fixed CSS
sizing allows us to do that.

The problem was that on Edge, flexbox apparently can grow canvas
elements, whereas on Chrome/FF the canvas width/height is respected. No
problem, we can set the width/height in CSS to fix it, which
conveniently allows us to also fix the low-res-on-retina-screens problem
at the same time.

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

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

* Implement paint-75: Add blank costume button

* add backdrop icon, use default name

thanks @fsih !

Replace the placeholder illustration.

* Add coming soon extensions to the library

This implements #1074

* don’t call `onSelect` when disabled

thanks @paulkaplan!

* add speech image to coming soon extension

* add feedback form

This implements #829

* small style updates to the stage buttons

from @carljbowman

* remove `stageSize` for now

We eventually need this, but not yet, and it’s causing lint failure.

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

Fixes #993