Resolve #3438897 "Avoid altering the theme registry"

• Factor out building sticky tasks to its own service.
• Simplify templates now that we're not potentially rendering two sets of local tasks within the one template.
• Add block plugin as one possible option for adding sticky tasks.
• Update config and add schema.
• Support optionally appearing on admin routes.
• Update JS to use once() and attach to the passed context.
• Update JS and templates to use data attributes rather than classes to identify important elements.
• Add JS dependencies to library.
• Add some tests. They're functional tests - it would be neat to also have a functional JS test that
  1. verifies links are shown and then hidden on clicking the toggle;
  2. verifies the links are on the right side of the screen.
• Add gitlab CI config to run the tests.
• Require PHP 8.1

Closes #3438897