Add next/previous backdrop options in backdrop block

Update chromedriver to the latest version 

Add new sprites 

* Add coming soon extensions to the library

This implements #1074

* don’t call `onSelect` when disabled

thanks @paulkaplan!

* add speech image to coming soon extension

Update to 2.8.0 manually because of greenkeeper test error.

Merge pull request #1147 from LLK/greenkeeper/scratch-vm-0.1.0-prerelease.1514561281-prerelease.1514561294

Update scratch-vm to the latest version 

Merge pull request #1146 from LLK/greenkeeper/scratch-vm-0.1.0-prerelease.1514558750-prerelease.1514558766

Update scratch-vm to the latest version 

Add drag mode block to make toolbox XML

Update scratch-blocks to the latest version 

Add new layer blocks to toolbox xml

Merge pull request #1144 from LLK/greenkeeper/scratch-vm-0.1.0-prerelease.1514558457-prerelease.1514558470

Update scratch-vm to the latest version 

Update scratch-blocks to the latest version 

Prevent deleting the last costume

* 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.

Update to most recent VM for broadcast updates

Update scratch-blocks to the latest version 

Broadcast Inputs

Merge pull request #1134 from LLK/greenkeeper/scratch-vm-0.1.0-prerelease.1514392948-prerelease.1514392961

Update scratch-vm to the latest version 

Update scratch-blocks to the latest version