Add alternate text for Scratch logo.

Wire up ask/answer components to the stage

Prevent switching tabs while one of these blocks is glowing from
crashing because it is given a new ID.

Use scratch-ll10n for extracting strings

Add music extension

Update enzyme-adapter-react-16 to the latest version 

Add aria label to language selector.

Add ask and answer blocks

Update react-style-proptype to the latest version 

Add Lego WeDo 2.0 extension to library

Add compatibility testing example page

Scroll to extension category only when loading from library

Load extension library data from a js file

Fix project json loading through storage

Allow loading the default project while offline

/ht @paulkaplan

Previously we tested the ProjectLoader helper object, but now all loading is confined to storage. Unfortunately I wasn't able to get enzyme to wait for the storage promise to resolve within componentDidUpdate, so I can't test the actual results of storage.load, just what it was called with.

eslint-plugin-import doesn't like these inline loaders.

I tried unsuccessfully to configure the recommended solution with `eslint-import-resolver-webpack`, but we should come back to this.

With this change we only use `xhr` in the `CostumeCanvas` component.