Add logo to header

Feel free to merge now, if you want. Can also wait until the
rest of the header pieces have been added.
Oh, good call. I forgot about `.u-js-only`. But we can't just use `.no-js` on the `noscript img` because that's the `html` class that gets removed when JS is available. I think mirroring that might be confusing. What about making a `.u-no-js-only` utility?