Refine look-and-feel

Created by: tysongach

• Clean up primary layout
  • Merge the header style component into main-content, which simplifies and reduces the Sass and gives us a BEM relationship for main-content__header and main-content__body to share styles.
  • Add some viewport-height-based padding to the bottom of the main content area so that the "save" button on forms is not shoved to the bottom of the screen.
  • Add a section element that wraps the body of the main content, to help make markup consistent across screens.
• Redesign tables
  • Slightly decrease font size of table content
  • Collapse table borders, to prevent gaps between cells
  • Define table styles as base HTML element styling
  • Refine table row focus outline
  • Prefix classes used for JavaScript interactions with js-
• Tweak colors
  • Use more saturated red and blue colors so that they pass WCAG contrast guidelines.
  • Related: #794

After this PR:

Before: