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.
Generated by [`style-loader`](https://github.com/webpack/style-loader/blob/master/index.js).