Animating submenus

Added a function to expand submenu items by toggling CSS classes. The
key to this is to write the HTML such that submenu items have
"collapsed" as a class to start with. That way, the JS function can
toggle both "expanded" and "collapsed", basically swapping states for
those elements.

Removed the setClosed() call from the links.on("click") event. It mucks
up all submenu functionality and I don't really like closing the menu
when that happens. The menu will still close when the top card is
clicked on.

Note that closing the main menu while a submenu is expanded will leave
the submenu expanded and just draw it upwards into the full stack
without pulling it into the center of the stack. This should be easily
fixable.

The flyout and flyin movement is handled via HTML classes and @keyframes
animations. Flyout starts with left: 0px; and ends at left: 210px;.
Flyin does that in reverse. Had to add the left property to the CSS
style rules so the elements stayed where the animations put them after
they finished.

I know about reversing the direction of animations but it wasn't working
so I just skipped that for now.