fix: typography of headlines and refactor

Fixes the jumbotron headline font size and letter spacing that was introduced shortly before launching the redesign. This should allow us to eliminate some hotfix tech debt introduced into Drupal, PHP and Hugo codebases.

Also cleans up the CSS a bit. featured-jumbotron-astro was removed as a class - the styles are now applied to featured-jumbotron. This should age better as the current look and feel is not part of the name. We will need to remove this unnecessary class from Hugo and PHP.

BREAKING: Also introduces a breaking change where .featured-jumbotron-dark is now .dark to be more consistent with how we've been handling dark sections recently.

Reverted the breaking change.