Reasons for creating this PR

@namedgraph reported in #1453 that there are problems with the UI language detection. The main problems were fixed in PR #1455, also by @namedgraph.

The WebController.guessLanguage method which is the core of language detection did not have unit tests, and as it was written, it was difficult to unit test it because it accesses cookies and HTTP header values directly.

This PR makes some changes to WebController.guessLanguage that make it easier to test, adds some basic unit tests that verify the main language detection cases, and then applies the fixes from #1455 on top.

Link to relevant issue(s), if any

• Closes Skosmos UI loads in Arabic #1453
• Supersedes UI language negotiation fix #1455 (includes the commits from that PR)

Description of the changes in this PR

• Upgrade willdurand/negotiation from 3.0.* to 3.1.* because only 3.1 is compatible with PHP 8.1
• Add handling of cookie values to the Request class
• Make WebController.guessLanguage use the Request class to access cookies and HTTP header values
• Implement unit tests for WebController.guessLanguage
• Fix the language value passed to in_array (from PR UI language negotiation fix #1455)
• Fix the language tags in skosmos:languages settings in the sample configs for Docker (from PR UI language negotiation fix #1455)

Known problems or uncertainties in this PR

The same fixes need to be applied to the skosmos-3 branch as well. I will do that in a follow-up PR.

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)