Reasons for creating this PR

The height of the gradient applied to the last tab in the sidebar on narrow screens is not set correctly. This PR fixes it.

Link to relevant issue(s), if any

• Part of Epic: Sidebar #1747

Description of the changes in this PR

Make the gradient height 100% of the parent nav-link element.

Known problems or uncertainties in this 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)