Resolve "Enterprise phase 4"

Why is this change being made?

Closes #9329 (closed)

Review app link

https://9329-enterprise-phase-4.about.gitlab-review.app/enterprise/index.html

Notes

• This MR introduces the following optional frontmatter attributes useable on all pages. The relevant code is in /source/includes/layout/head.html.haml.
  • disable_default_styles
  • disable_fontawesome
  • disable_tiny_slider
• This MR introduces the following optional frontmatter attributes useable on all pages using the default layout. The relevant code is in /source/layouts/default.html.haml.
  • disable_handbook_styles
• This MR introduces the following performance optimizations
  • Disables loading the mermaid javascript except on pages using the handbook or markdown templates.
• This MR introduces new utility sizes, xxs (10px) and xxl (150px).
• To significantly improve performance, this MR removes some of the animations in the mobile hamburger navigation menu.

Observations:

• Many of our sass color variables are duplicates or near-duplicates that should be consolidated.
• We're loading FontAwesome twice for very little impact. Once for version 5+ on our side, and once for version 4.6 via our search provider.
  • It was only used in the footer icons. If you're using the disable_fontawesome option, make sure you apply related SVG icons from the footer similar to the 2020base.scss file.