Adjust header and search box to work on mobile. Closes #69

Added a breakpoint, using mobile first and adding a breakpoint for greater
than 600px. This adds a colored background for region so it overlaps the
other text gracefully. Makes title text smaller and removes top margin.
Moves search box down and slides other content down accordingly.

Heard about your site via openhatch.org and saw this issue there. Let me know if there's anything special I need to do beyond a PR. Thanks!!