Make shortcuts "u", "[", and "]" more general

The previous implementation had custom treatment for these shortcuts.
This commit introduces a more general approach where the
`data-keyboard-shortcut` attribute is added to elements that listen to
the "u", "[", and "]" shortcuts.
Add [a]ccept and [r]eject keyboard shortcuts

Now that we have a more general way of configuring shortcuts, we can
easily add "a" and "r" to accept and reject a snapshot.