USAGOV-1989: Page intro as fallback for meta description on wizards

[DaleMHFrey]

https://cm-jira.usa.gov/browse/USAGOV-1989

Description

When the mate_description is empty, we use the intro field instead for the meta-tag output.

Type of Changes

• New Feature
• Bugfix
• Frontend (Twig, Sass, JS)
• Drupal Config (requires "drush cim")
• New Modules (requires rebuild)
• Documentation
• Infrastructure
  • CMS
  • WAF
  • WWW
  • Egress
  • Tools
  • Cron
• Other

Testing Instructions

• Edit a term like https://localhost/taxonomy/term/67/edit and empty its Meta-Description field.
• Make sure there is something in the Intro field.
• Save, and load the page on the front end. Inspect the source-code of the front-end page (view page source)
• Look for the <meta tag that contains the "description" of the page. The value for the meta-tag should be the contents of the Intro field now

Post PR Approval Instructions

Follow these steps as soon as you merge the new changes.

1. Go to the USAGov Circle CI project.
2. Find the commit of this pull request.
3. Build and deploy the changes.
4. Update the Jira ticket by changing the ticket status to Review in Test and add a comment. State whether the change is already visible on cms-dev.usa.gov and beta-dev.usa.gov, or if the deployment is still in process.