Group Settings Default initial branch h4 expand

Group Settings / Repository Settings / Default initial branch name
headers (h4 elements) do not expand/collapse on mouse-click / tap, but
only using the Expand/Collapse button. This may seem inconsistent to
the user.

This MR adds classes settings-title, js-settings-toggle and
js-settings-toggle-trigger-only to the h4 headers in Group Settings /
Repository Settings / Default initial branch name. This enables the
expand/collapse behavior on on-click/on-tap for the h4 headers.

Changelog: changed

app/views/groups/settings/repository/_initial_branch_name.html.haml

 %section.settings.as-default-branch-name.no-animate#js-default-branch-name{ class: ('expanded' if expanded_by_default?) }
   .settings-header
-    %h4
+    %h4.settings-title.js-settings-toggle.js-settings-toggle-trigger-only
       = _('Default initial branch name')
     %button.gl-button.js-settings-toggle{ type: 'button' }
       = expanded_by_default? ? _('Collapse') : _('Expand')