Fix dependency issues with usfs and starter app

[dmethvin-gov]

In trying to get the examples working with codesandbox.io I found that we have some incorrect dependency setups that don't show with local installs. USFS lists USWDS as a peerDependency but the starter app doesn't list it as a dependency as it should. The most obvious result of this issue is that images that should be on the form (like the arrows in select boxes) don't show.

Also, if you want to build a production copy of the starter app and run npm install --production it's not possible to build the app because a production build requires webpack and several of the loaders. We should either document that or load webpack so that we could have a prepare do the production build.

[dmethvin-gov]

This is related to #66 and usds/us-forms-system-starter-app#26. Regardless of how the other details are resolved I think it makes the most sense for the app to be in control of pulling in the USWDS dependency and the widgets of USFS treat it as a peerdep the way we do today.

[dmethvin-gov]

The immediate problem was fixed by usds/us-forms-system-starter-app#26 and the "how do we handle styles/images/fonts" part can be hanadled in #66.