- 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.

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.

This is for https://github.com/LLK/scratch-paint/pull/194, making the selector area smaller on tablet so as to create more space for the paint editor