Merge pull request #2407 from paulkaplan/backpack-step-5

package.json

src/components/asset-panel/selector.jsx

src/components/backpack/backpack.css

src/components/backpack/backpack.jsx

src/containers/backpack.jsx

src/containers/costume-tab.jsx

src/containers/sound-tab.jsx

src/lib/backpack-api.js

src/lib/backpack/costume-payload.js

src/lib/backpack/jpeg-thumbnail.js

src/lib/backpack/sound-payload.js

src/lib/backpack/sound-thumbnail.jpg