Reasons for creating this PR

This PR implements most of the remaining SEO functionality from #1533 :

• vocabulary description on vocab home page
• og:site_name on all pages
• og:type="website" on all pages
• og:url and link rel=canonical on all pages
• drop twitter:card (shouldn't be needed since twitter/X falls back on og:type)

In addition, there is a bug fix to the Request.getLangUrl method that broke when the Skosmos 3 code was restructured by moving backend code under src/. This fix could have been a separate PR but since the bug was causing problems for the canonical URL generation, I decided to fix it here in this PR. The fix is in a separate commit 9bdb894 .

Link to relevant issue(s), if any

• part of Epic: Search Engine Optimization #1533

Description of the changes in this PR

• add VocabularyConfig.getDescription() method + PHPUnit test
• add new metadata elements to base-template.twig (and drop twitter:card)
• add blocks in page type templates to populate the metadata
• add cypress tests
• fix bug in Request.getLangUrl() method + correct the PHPUnit tests accordingly

Known problems or uncertainties in this PR

None so far!

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)