Rewrite collapsibleNav as LESS file

* Rewrote css file as a LESS file for better maintainability
* Make use of core embed mixin
** Add .background-image-svg mixin for rendering svgs as background images

Change-Id: I72a66553d6ecee5018d4efaabfd0fa0df6c4135f
Merge "Rewrite collapsibleNav as LESS file"