Project Settings CI/CD Variables Header Expand/Collapse

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.

app/views/ci/variables/_header.html.haml

 - expanded = local_assigns.fetch(:expanded)
 
-%h4
+%h4.settings-title.js-settings-toggle.js-settings-toggle-trigger-only
   = _('Variables')
 
 %button.btn.gl-button.btn-default.js-settings-toggle{ type: 'button' }

changelogs/unreleased/205484-03-project-settings-headers-cicd-variables.yml

+---
+title: Project Settings CI/CD Variables header expands/collapses on click / tap
+merge_request: 54117
+author: Daniel Schömer
+type: changed