Reasons for creating this PR

Fix a bug where changing the UI language after a partial page load could load the wrong page, see #1848.

This PR adds one more step to the partial page load: updating the topbar navigation links by copying them from the loaded concept page into the current page DOM. I also added a Cypress E2E test to verify that it works.

Link to relevant issue(s), if any

• Closes Changing UI language after partial page load opens wrong page #1848

Description of the changes in this PR

• add updateTopbarNav function to partial-page-load.js
• add Cypress test to verify

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)