UI changes for responsive layout on small screens

[bauhouse]

This pull request addresses a number of issues:

• Site header is too wide on small screens #44
• Footer elements overlap on small screens #45
• The .get-symphony class is too wide for small screens #46
• The background on the .network-nav doesn't work so well for small screens #47

Happy New Year! :D

[nilshoerrmann]

Thanks Stephen. And Happy New Year to you and your family, too!