Make search-based select fields keyboard navigable

[timriley]

Right now I can manage to get a search-based select field focused with the keyboard, and hitting will pop up the search box, but the search box isn't focused at that point, so I can't start typing.

After I've manually focused the search box, once I've typed something and see results, I want to be able to use the arrow keys to move up and down through the results. I realise I can use the tab key to do this, but I feel like people will reach for up and down arrow keys instinctively.