SEO: add description metadata to concept page

[osma]

Reasons for creating this PR

Adding one final missing piece of SEO functionality: description metadata on the concept page.

The description is translated using the normal translation mechanism. The current translations are:

• Finnish: Käsite xxxx sanastossa yyyy
• Swedish: Begreppet xxxx i vokabulären yyyy
• English: Concept xxxx in vocabulary yyyy
• Northern Sámi: Doaba xxxx sátnerájus yyyy

where xxxx is the prefLabel of the concept and yyyy is the title (not the short name) of the vocabulary, as specified earlier in #1533.

Link to relevant issue(s), if any

• part of Search Engine Optimization for Skosmos 3.0 #1533

Description of the changes in this PR

• add description block to concept page twig template
• add translations (fi, sv, en, se) needed to generate the translation
• add cypress test to verify that it works

Known problems or uncertainties in this PR

None

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)

[joelit]

LGTM - works as expected!