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.

Also assigns class definition directly into `module.exports` and adds `displayName` for jsx parser because it doesn't have a local variable to look at.
Talk to me about this. Usually the JSX transform applies the displayName automatically. What's making us need to define this manually?