Project Settings General Headers (h4 elements) expand/collapse on
mouse-click / tap, like using the Expand/Collapse button. Other h4
headers in other Project Settings pages do not expande/collapse on
mouse-click / tap, but only using the Expand/Collapse button. This may
seem inconsistent to the user.

The h4 headers of Project Settings General get the expand/collapse
behavior through classes settings-title, js-settings-toggle and
js-settings-toggle-trigger-only.

This MR adds classes settings-title, js-settings-toggle and
js-settings-toggle-trigger-only to the h4 header of Project Settings
CI/CD Variables for GitLab FOSS. This enables the expand/collapse
behavior on mouse-click / tap for the h4 headers.