Seems there's fundamental issue where the Blockly repo is compiling scss -> css at build time, but `appOptions.baseUrl` is set dynamically at blockly initialization time. Don't think there's a great way to do cross-dashboard/blockly scss template interpolation. If we ended up needing it, we could pull the css out into dashboard and have Rails configure via its scss templating.

For the purposes of getting a single point of truth for the image paths within the blockly scss, the most canonical way would be to upgrade grunt-sass and use the new `imagePath` build option added in `0.12.1`: https://github.com/andrew/node-sass#imagepath and change our urls to use the `image-url()` helper. Would be better to do as a separate branch, though.