- 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/1103

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

Localizes the pen extension:
- loads all pen locale information - may want to consider dynamically loading locales later.

  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.