Retrieve assets from the VM rather than storage

src/components/asset-panel/selector.jsx

src/components/sprite-selector/sprite-list.jsx

src/components/target-pane/target-pane.jsx

src/containers/costume-tab.jsx

src/containers/sprite-selector-item.jsx

src/containers/stage-selector.jsx

src/containers/watermark.jsx

src/lib/backpack/costume-payload.js

src/lib/backpack/sound-payload.js

src/lib/backpack/sprite-payload.js

src/lib/get-costume-url.js