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

Fixes https://github.com/LLK/scratch-gui/issues/1064

* Implement GH-830: Add coming soon tooltips

This implements coming soon tooltips for:
* the small stage size
* save, load and language

It also creates the component that can be copied over into the paint editor for use there as well.

Lastly, this does a small refactor on the full screen implementation to include multiple small screen sizes too.

- New icons
- Play button moved to prominent location
- Styling normalized with costume editor

  1. Removed duplication of code in stage component to avoid future
     potential divergence. Now just selectively applying styling based
     on states of zoomed, colorPicking, and question.

  2. Fixed bottom border getting chopped in zoomed mode by adding margin
     and adjusting height/width accordingly.

  3. Removed shouldComponentUpdate from stageHeader as suggested from code
     review. Added ignore lint warning suggesting change to stateless function.