UHF-7841: Add Video prefix for iframe title

[xkhaven]

UHF-7841

What was done

• Added "Video: " prefix for remote videos iframe title for accessibility improvements

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-7841_Video_accessibility_updates
• Update the Helfi Platform config
  • composer require drupal/helfi_platform_config:dev-UHF-7841_Video_accessibility_updates
• Run make drush-updb drush-cr

How to test

• Add new remote video paragraph and check that "Assistive technology title" has guidance updated for not to use prefix "Video: " on title
• Remote video's iframe title has "Video: " prefix attached
• Check that code follows our standards

Designers review

• This PR does not need designers review

Other PRs

• UHF-7841: Updated video title guidance for editors drupal-helfi-platform-config#627