Reasons for creating this PR

This PR adds support for new SEO configuration options skosmos:serviceDescription, skosmos:aboutDescription and skosmos:feedbackDescription which make it possible to set a description for the landing, about and feedback pages, respectively. The values are multilingual. If not set, there will be no description metadata on the respective page.

Link to relevant issue(s), if any

• Part of Epic: Search Engine Optimization #1533

Description of the changes in this PR

• add support for skosmos:serviceDescription, skosmos:aboutDescription and skosmos:feedbackDescription settings in GlobalConfig
• use the settings to add description metadata to the HTML templates of the landing, about and feedback pages
• PHPUnit and Cypress tests

Known problems or uncertainties in this PR

There is no Cypress test for the case when one of the descriptions is not set in the configuration and thus should not be displayed. Should there be? The problem here is that this would require a separately configured Skosmos instance with a different configuration that omits some or all of the description settings. We currently only use one Skosmos instance for running Cypress tests, using the tests/testconfig.ttl configuration file.

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)