Updates to Webpack

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

Changes build process from gulp to webpack where possible.

Splits dropdown.js into three separate files in a components folder
that uses require to bring in necessary dependencies.
Did you research if there is a way to hide these absolute paths from the output? My paths show up in `git diff` after I `gulp`.