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.
Same as comment in [ddm-dropdown](https://github.com/deseretdigital-ui/ddm-dropdown/pull/4#discussion_r20475212)