Fauxton: fix scrollbar-issue - COUCHDB-2234

- Try to honor borders in calculating the width
- Adjust magic numbers to new widths of the left area
- Refactor the header/api-url section
- Removing the maximum widths of 1400px and 1500px, which caused
  vertical scrollbars to appear at 1400px (which is on top of the
  page content, some hundred pixels away from the left side on
  large screens) on a system like windows

COUCHDB-2234