Improvements to error state: input outline shows with or without focus, and added new error div which shows if there's an error, but goes away if the user starts changing what they had submitted in the input. Removing a lot of code surrounding the tooltip which has now become unnecessary.

Import input now shows error state w/orange border around input and tooltip with static error message.

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

Update gui to use getter for target costumes

Move active tab state into a reducer

Greenkeeper updates 2 22 18

Closes #1450

Closes #1461

Closes #1445

Remove enableExtensions prop

Fixed issue #1409

Update scratch-paint to the latest version 

Fix issue 1212

Detect wheel event on canvas and post data to VM

update my fork

Apply a drag threshold to make clicking sprites easier

Update dependencies to enable Greenkeeper 

- Call scrollToCategoryByName('My Blocks') after custom block is created
- Cannot assume custom blocks section will be last category
- May pose issue during localization efforts