Merge pull request #3581 from paulkaplan/backpack-code--rebased

src/components/backpack/backpack.jsx

src/components/blocks/blocks.css

src/components/blocks/blocks.jsx

src/containers/backpack.jsx

src/containers/blocks.jsx

src/lib/backpack-api.js

src/lib/backpack/code-payload.js

src/lib/backpack/code-thumbnail.js

src/lib/drag-constants.js

src/lib/drop-area-hoc.jsx