LP#1778972: Tweaks and fixes to Bootstrap 4 template

LP#1778972: Tweaks and fixes to Bootstrap 4 template

- Add "aria-hidden" to Font Awesome icons as recommended by Font Awesome for accessibility.
- Minor changes to the color scheme.
- Some adjustments to the way alignment is defined: Bootstrap 4 classes like "text-center"
  can be an impediment to customization because they use "!important." I've removed many
  instances of such classes in favor of putting alignment in CSS.
- Ajustments to responsive behavior. I've made some changes to grid class definitions
  to improve flexibility of layouts at various browser widths.
- Whitespace and other typo corrections. I've worked to remove trailing whitespace and
  replace tabs with spaces.

Signed-of-by: Owen Leonard <zivot@zivotdesign.com>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>
Signed-off-by: Jane Sandberg <sandbej@linnbenton.edu>