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.
I found that assigning it directly to `module.exports`, the JSX transformer ends up with `displayName: 'exports'`.