Migrate loading spinners

Migrate loading spinners to be
design system compliant.

Changelog: changed

app/views/dashboard/_activities.html.haml

@@ -6,4 +6,4 @@
 
 .content_list
 .loading
-  .gl-spinner.gl-spinner-md
+  = gl_loading_icon(size: 'md')

app/views/dashboard/groups/_groups.html.haml

 .js-groups-list-holder
   #js-groups-tree{ data: { hide_projects: 'true', endpoint: dashboard_groups_path(format: :json), path: dashboard_groups_path, form_sel: 'form#group-filter-form', filter_sel: '.js-groups-list-filter', holder_sel: '.js-groups-list-holder', dropdown_sel: '.js-group-filter-dropdown-wrap' } }
-    .loading-container.text-center.prepend-top-20
-      .gl-spinner.gl-spinner-md

app/views/groups/_activities.html.haml

@@ -6,4 +6,4 @@
 
 .content_list
 .loading
-  .gl-spinner.gl-spinner-md
+  = gl_loading_icon(size: 'md')

app/views/projects/merge_requests/show.html.haml

@@ -83,7 +83,7 @@
 
     .mr-loading-status
       .loading.hide
-        .gl-spinner.gl-spinner-md
+        = gl_loading_icon(size: 'lg')
 
 = render 'shared/issuable/sidebar', issuable_sidebar: @issuable_sidebar, assignees: @merge_request.assignees, reviewers: @merge_request.reviewers, source_branch: @merge_request.source_branch