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

* 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.

Alphabetize all the things

Remove target pane button and actions

Add back putting the sound library in the ui