WIP branch for teaching the designer about components that live at arbitrary URLs.

Still to do:

- [x] fix the UI to be less ugly
- [x] explore whether to special case github pages for example (in particular would require proxying to get around CORS)
- [x] explore whether JSBIN would work, and whether special casing is necessary (nope -- won't work w/o customizing jsbin to not decorate published html)
- [x] code review from @simonwex