Add more UI states (e.g. differentiating between no access, permission granted but video not loaded, video loaded). CSS cleanup.

Move runtime/renderer related code back into the vm io device. Refactor ModalVideoManager to use the video provider.

Refactor Video IO

Protect against showing costume tab for target that does not exist.

Revert "Bring back "Updates to preview modal for release""

Prevent crashes resulting from the editing target not being on the target list.

chore(package): update scratch-blocks to version 0.1.0-prerelease.152…

Closes #1913

Make sound editor buttons unselectable

Post keyboard event.key

Use special method for downloading blobs in Edge

chore(package): update chromedriver to version 2.38.2

Prevent category menu jump on sprite switch

Update scratch-blocks to the latest version 

Separate video provider and modal video manager. Provide hooks in modal video manager for calling functions during stages in enableVideo.

Update scratch-storage to the latest version 

Update scratch-render to the latest version 

Bring back "Updates to preview modal for release"

Revert "Revert to pre-bitmap paint editor for deploying"

Add a simple smoke test for the how-to library

Allow more mime types to be processed.

Firefox and Safari both process mp3s as having audio/mpeg, and a variety of different audio/wav* possibilities.