Apply a drag threshold to make clicking sprites easier

Update dependencies to enable Greenkeeper 

Fix numerous draggable images in the GUI

Use the same drag threshold as from the blocks. It may need to be dialed
in further, but testing will be needed.

Update react-modal to the latest version 

Images are draggable by default, but when used within buttons or as
clickable elements, they should not be draggable because starting a drag
accidentally prevents the click event.

Update react-style-proptype to the latest version 

fix not now to redirect to scratch.mit.edu

Merge pull request #1421 from LLK/greenkeeper/scratch-vm-0.1.0-prerelease.1518098842-prerelease.1518098861

Update scratch-vm to the latest version 

Lower long and double click timing to 400ms to match scratch 2

Update paint version for bugfixes

Fix the broken box shadow to make inputs pop more on focus

Revert "Remove duplicate computer beeps"

Remove duplicate computer beeps

Update react-modal to the latest version