Keyboard nav in the site selectors

I reflexively and habitually hit down arrow when searching site selectors only to be stymied by lack of keyboard navigation. This is a common, oft repeated interaction in my daily routine. The habit is reinforced by other sites in my daily flow that do support down + enter.

 

https://github.com/Automattic/wp-calypso/issues/8

#8-gh-wp-calypso, #chrome, #desktop, #keyboard-navigation, #macos, #multisite, #site-selection