Use grunt for CSS

- [ ] Ready for merge

Uses grunt for minifying the main CSS file for jsbin. Based on original PR #1263 by @daniel-hug.

## Tasks

- [x] Ensure production files are not committed
- [ ] How are themes loaded into the client side - note that the user will typically change this from the editor settings, so we could use the server side `req.session.user.settings.editor.theme` to load their theme.
- [ ] Update CSS based on master (this branch, at time of writing, is behind)

For now, this doesn't need to apply to the editor or preferences pages (which include a few CSS files, but they're low traffic and are loaded from different points)