Create header

[dgading]

Create a new header using the latest version of React Router.
Add links for:

• Home
• Datasets
• Publishers
• About
• API
• 404

Add dkan logo, dkan site name, and dkan slogan. Design should look similar to the current DKAN demo site. Since we are moving away from React Strap, mobile styling can be referenced in the CMSDS component library possibly.

[dgading]

#158 adds a basic header with some mobile styling. We will probably need some ux/design and accessibility passes in the future to clean it up for release.