adding css transitioned collapse / expand to togglers

We tend to use expanding, collapsing toggler like widgets from 3rd party libraries because the display:none » block toggler isn't user friendly. I think the collapsing transition can be useful in many cases.

This feature follows declarative principles. I tested it and it's non obtrusive. The JS and scss only kicks in if <code>data-ur-collapsible='enabled'</code>