Reasons for creating this PR

Overwhelmingly long lists of property values on the concept page should be truncated, see #1795. This functionality exists in Skosmos 2, and this PR implements the same feature for Skosmos 3.

I looked at the jQuery-based implementation of this functionality in Skosmos 2 and adapted the same logic for Skosmos 3, but using vanilla JS instead. There is also a Cypress test that verifies that it works both in the initial page load and partial page load cases.

Example of the result (on the page http://localhost/Skosmos/ykl/fi/page/38.55 ):

Link to relevant issue(s), if any

• Closes truncate property value lists that are longer than set number of items with "show all X values" button #1795
• part of Epic: Concept Page #1484

Description of the changes in this PR

• add vanilla JS code that truncates long lists of property values (by adding CSS classes to the elements), triggered on initial page load & partial page load
• add CSS rules that hide/reveal property values as appropriate
• add Cypress tests to verify the above

Known problems or uncertainties in this PR

• The truncation limit is hardcoded at 15, as in Skosmos 2. It would take more work to make it configurable.
• This PR only truncates regular property values, e.g. skos:narrower. It does not truncate the "terms in other languages" section, unlike Skosmos 2, where also these were truncated. In Skosmos 3, the DOM structure of the "in other languages" section is very different than regular properties, so the same code doesn't work for both.

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)