got simple board chooser working, which is based on board URL
change layout a little. Still seeing a bug where Go button is clickable, even though chooser is not navigable
fix bug: was not updating navigable() because it was not defined as obervable. Also, made chooser update after each keypress
only allow board ids in chooser that are the right length
only show chooser when there is not a board id navigated to
remove previously-shown cards when going back to main app page