Header and Nav merged together, TBM module added for Nav.

Uses https://www.drupal.org/project/tb_megamenu to create a basic megamenu nav.

Relies on https://gitlab.com/drupal-infrastructure/sites/drupalorg/-/merge_requests/31 for configuration of the module, and updates to configuration as a result.

Closes #3473419

Testing steps

1. Checkout https://gitlab.com/drupal-infrastructure/sites/drupalorg/-/merge_requests/31
2. Checkout this branch of bluecheese
3. Run composer recreate
4. Clear cache
5. Login as admin
6. Run structure sync to pull in main menu links for demonstration
7. View any page and see the header navigation.

Known issues / Work remaining

• Submenus have a basic styling only
• Block insertion is currently not tested in this MR
• Mobile navigation is the basic styling that comes from TBM module
• No link on header logo
• Search / user menu icons are not present
• Styleguide is not representative of the styling on front end for the menu, and currently is not functional for the Nav component