Honor className Prop on All Components and Eliminate Content Wrapper

Merge className prop in instead of overriding in Collapsible,
CollapsibleGroup, CollapsibleHead and CollapsibleBody components.
Update index to prove it works.

Eliminates .ddm-dropdown__content element by summing height of children.
Merge pull request #9 from deseretdigital-ui/className

Honor className Prop on All Components and Eliminate Content Wrapper