Keyboard shortcuts

[cespare]

We'll need a keyboard shortcut help dialog of some kind as well.

Some initial ideas: (ctrl on linux/windows, cmd on mac)

• j/k to scroll the message pane up and down
• ctrl-u/d to scroll up and down by half a page
• G to scroll to the bottom
• ctrl-n/p to switch next/previous channel
• ? open keyboard shortcut help
• enter to focus the text box
• Typing anything that isn't a shortcut on the page also focuses the text box?

[mdietz]

Duplicate of #57