gen-regions: Update to site styling

This updates the gen regions page to have a consistent styling with the
rest of the site, and an improved look-and-feel.

Closes #22.
Closes #23.
Closes #36.
gen-regions: Fix headers

Apply identical headers to all gen-regions sources
assets/gen-regions.css: Remove unused selector
gen-regions: Move into contained subdirectory
assets/gen-regions.js: Use window.onload for init
Merge branch 'iss23'