UHF-8689: Captions for illustrations on special pages

[teroelonen]

UHF-8689

What was done

• Add reusable template and styles for special page illustrations.
• Convert error pages, maintenance page and login page to use it.

How to install

• Make sure your instance is up and running on latest dev branch.
  • git pull origin dev
  • make fresh
• Update the HDBT theme
  • composer require drupal/hdbt:dev-UHF-8689
• Run make drush-cr

How to test

• Go to /user/login and make sure that the illustration still looks good on the page (compare it to for example some production page). Make sure that there is now also the name of the creator of the illustration.
• Do the same check for /asfdsgdfhsdh page which is the 404-error page.
• Next do the same check for /admin when you are not logged in which is the 403-error page.
• Lastly log in to the site and put it to maintenance mode and go see the page in incognito window. Check the same thing for the illustration there. You can compare this for example to some other instance on your local that you can also put to maintenance mode.
• Check that code follows our standards,

Continuous documentation

• This feature has been documented/the documentation has been updated
• This change doesn't require updates to the documentation