New website template/design

[GUI]

• Update our site to match the new Data.gov template and website design.
• Revamp some of our website content to try and simplify things:
  • Prioritize the about page for agencies.
  • Remove links to our API key signup form (since ideally all users will be signing up on agency pages, since we've found that's less confusing for users).
  • Remove links to individual agency documentation pages, since we're trying to get away from needing to maintain that list and be more of an invisible service agencies can utilize.
  • Surface links to some pages that we previously shared with agencies or specific users, but otherwise weren't linked to (like the agency manual or our metrics page).
  • Remove various defunct pages.
• Various updates to dependencies, build process, and linting/style (eg, running everything through prettier).