Apply GitLab UI button styles to buttons in app/views/admin/labels directory

app/views/admin/labels/_form.html.haml

@@ -27,5 +27,5 @@
       = render_suggested_colors
 
   .form-actions
-    = f.submit _('Save'), class: 'btn btn-success js-save-button'
-    = link_to _("Cancel"), admin_labels_path, class: 'btn btn-cancel'
+    = f.submit _('Save'), class: 'btn gl-button btn-success js-save-button'
+    = link_to _("Cancel"), admin_labels_path, class: 'btn gl-button btn-cancel'

app/views/admin/labels/_label.html.haml

 %li.label-list-item{ id: dom_id(label) }
   = render "shared/label_row", label: label.present(issuable_subject: nil)
   .label-actions-list
-    = link_to edit_admin_label_path(label), class: 'btn btn-transparent label-action has-tooltip', title: _('Edit'), data: { placement: 'bottom' }, aria_label: _('Edit') do
+    = link_to edit_admin_label_path(label), class: 'btn gl-button btn-transparent label-action has-tooltip', title: _('Edit'), data: { placement: 'bottom' }, aria_label: _('Edit') do
       = sprite_icon('pencil')
-    = link_to admin_label_path(label), class: 'btn btn-transparent remove-row label-action has-tooltip', title: _('Delete'), data: { placement: 'bottom', confirm: "Delete this label? Are you sure?" }, aria_label: _('Delete'), method: :delete, remote: true do
+    = link_to admin_label_path(label), class: 'btn gl-button btn-transparent remove-row label-action has-tooltip', title: _('Delete'), data: { placement: 'bottom', confirm: "Delete this label? Are you sure?" }, aria_label: _('Delete'), method: :delete, remote: true do
       = sprite_icon('remove')

app/views/admin/labels/index.html.haml

 - page_title _("Labels")
 
 %div
-  = link_to new_admin_label_path, class: "float-right btn btn-nr btn-success" do
+  = link_to new_admin_label_path, class: "float-right btn gl-button btn-nr btn-success" do
     = _('New label')
   %h3.page-title
     = _('Labels')