Should help with smaller screen sizes, where the ratio is different (like some chromebooks)

Initial SB2 Import UI (button in preview modal + import modal that can load projects via url). Preliminary validation on url, still needs error handling.

Images are draggable by default, but when used within buttons or as
clickable elements, they should not be draggable because starting a drag
accidentally prevents the click event.

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

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

* 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.

* 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.

I changed the color of the bottom of the tab when selected from white
to the background color of the block chooser.
The main reason for the change was that the white bottom on the tabs
did not blend well with the boundary between gray/blue and white when
selected.  It has been changed so that there is a line that is lighter
than the regular border color which signifies that the tab is selected
and gives it separation from the white content but doesn’t separate the
content too far from the tab as a darker color would do.

Changed existing string ids to use the convention `<repo>.<component>.<identifier>`

Alphabetize all the things

  Commit fceba4e8 introduced new sizing behavior for the stage in
  order to fit everything in a 1024 width. Adopted this same behavior
  for the StageHeader component.

To show the "Add Extension" button, you must now add `?extensions` to
the URL used to load the GUI.

  1. Fixed issue with sprites not being dragable when stage zoomed.

  2. Fixed issue with coordinates of mouse position being wrong when
     stage is zoomed. This was caused by the renderer not being
     resized when zooming/unzooming.

  3. Refactored zoom control to separate react component/containter
     with global reducer for isZoomed, which is passed to the stage
     container. This allows for the zoom control to zoom the stage
     menu and stage as seperate containers.

  4. Added icons zoom/unzoom for stage zoom control. Moved icon
     position to be part of stage menu floating to right of start/stop
     buttons.

Add class name props to the components, and fix positioning for the green flag to be reusable.

It wasn't ever necessary. Still allow configuring this path in the GUI in case of different paths to static files for scratch-blocks.