Updates to Webpack

Reconfigures ddm-collapsible to compile all the assets used by
collapsibles into a single JavaScript file. This includes the necessary
CSS. It also allows us to distribute the file in UMD format.

Changes build process from gulp to webpack where possible.

Moves gulp tasks into their own individual files.
Ah, shoot. I pulled these transition helpers into another library for something else but forgot to update this component.

https://github.com/deseretdigital-ui/transition-helpers

Not sure how you want to pull those in.