Fix diacritic letters in alphabetical index (Skosmos 3)

[osma]

Reasons for creating this PR

Fix the problem of diacritic letters (ÅÄÖ...) in Skosmos 3. It was already fixed for Skosmos 2 in PR #1575; this PR includes the fixes from that PR and also adds a Cypress test to verify it on the UI level.

Link to relevant issue(s), if any

• Closes Alphabetic tab doesn't work with certain diacritic letters #1574

Description of the changes in this PR

• switch input filtering of server constants from FILTER_SANITIZE_FULL_SPECIAL_CHARS to FILTER_SANITIZE_ADD_SLASHES
• add PHPUnit test to verify server constant sanitizing
• add Cypress test to verify that diacritic letters work in alphabetical index

Known problems or uncertainties in this PR

n/a

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)

[sonarcloud]

Quality Gate passed

The SonarCloud Quality Gate passed, but some issues were introduced.

1 New issue
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1e17ce9) 70.54% compared to head (bf68c52) 70.54%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1580   +/-   ##
=========================================
  Coverage     70.54%   70.54%           
  Complexity     1644     1644           
=========================================
  Files            32       32           
  Lines          4315     4315           
=========================================
  Hits           3044     3044           
  Misses         1271     1271           

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[joelit]

Looks good. The chosen filter seems like a right call - the diacritics in initial letters seem to work, and it does not break special characters.