Prevent left hand menu from "falling off the edge" of the app

When the app is sized so that it doesn't fill the width of the window, the left-hand menu slides outside the bounds of the app.

This fixes it by setting "overflow-x: hidden" on the main container for the app.

I also tweaked the animation slightly, as it was too slow and the wrong way round (i.e. it did ease-in when ease-out seems more natural to me).