Update scratch-blocks to the latest version 

chore(package): update scratch-l10n to version 3.1.20181220222259

Removes the three languages from the menu that still need to be reviewed

chore(package): update scratch-vm to version 0.2.0-prerelease.20181221030743

Closes #4174

translate form validation string

Add telemetry opt-in/opt-out dialog

Closes #4173

More library updates

The playground now displays this on startup when in Scratch Desktop
simulation mode.

The file uploader file input should accept .sb files as well.

Update deps to preserve spaces on stage

Implement list import and export from files

On modal mount, push history if current history state doesn't match this modal's id

Reduce analytics events

Use new sentry API provided by www, and use analytics ID from www

refactored font loading state to be in redux

Make remixing/saving faster by marking all assets as clean to start.

Update scratch-vm to the latest version 

Ignore projectChange updates from the VM when not in editor mode

Do not remove the window beforeunload handler in componentWillUnmount.

This is because when switching from project to editor view, this component gets unmounted and then remounted, but the componentDidMount seems to happen before the componentWillUnmount. Because it is dealing with a global variable, it is unsafe to always unset the beforeunload when unmounting.