Make `shortcutKeys` use number keys

I previously used strings because I didn't know that you could use
numbers as keys.

I also sorted the shortcuts alphabetically.
Only `preventDefault` if shortcut is used

Previously, we would prevent the default key press behavior for any key
hit. Now we only prevent if the key hit is a shortcut key (or any of the
hard-coded keys) and there was a shortcut present on the page.

I also removed an unnecessary `break`.
Make `.keyboard-focused` a variable

We had hard-coded this string in a number of places. Making it a
variable prevents us from making naming mistakes.