Migrate loading spinners

Migrate loading spinners to be design
system compliant.

Changelog: changed

app/views/groups/imports/show.html.haml

@@ -4,7 +4,7 @@
 .save-group-loader
   .center
     %h2
-      %i.loading.gl-spinner
+      = gl_loading_icon(size: 'md')
       = page_title
     %p
       = s_('GroupImport|Please wait while we import the group for you. Refresh at will.')

app/views/projects/merge_requests/creations/_new_submit.html.haml

@@ -48,4 +48,4 @@
 
   .mr-loading-status
     .loading.hide
-      .gl-spinner.gl-spinner-md
+      = gl_loading_icon(size: 'md')

app/views/shared/milestones/_tab_loading.html.haml

-.text-center.gl-mt-3
-  .gl-spinner.gl-spinner-md
+= gl_loading_icon(size: 'md', css_class: 'gl-mt-3')