Refactor: Convert linkUrlFilter to a Twig extension class

[osma]

Reasons for creating this PR

This makes the controller code a bit cleaner. It is also necessary to make it possible to extract messages from Twig templates in PR #1648 .

See #1653

Link to relevant issue(s), if any

• Closes Twig link_url filter into its own class #1653

Description of the changes in this PR

• move linkUrlFilter from Controller to its own Twig extension class

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

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[miguelvaara]

The tests pass, the code changes look meaningful and the final result works as desired. Ready to merge!