Focus search field when `/` is pressed

[wincent]

May be a bit involved because at the moment there is no search field on most pages (clicking on "Search" in the nav takes you to /search, at which point you see the field).

So, we can either build an in-nav search (a larger lift, as we don't even have as-you-type feedback in place on the existing search page), or we could just have / take you to the search page and focus the field.