Add gitlab button class

As per https://gitlab.com/groups/gitlab-org/-/epics/4001

ee/app/views/shared/_namespace_storage_limit_alert.html.haml

@@ -12,7 +12,7 @@
 
 - style = namespace_storage_alert_style(alert_level)
 - icon = namespace_storage_alert_icon(alert_level)
-- purchase_link = purchase_storage_url if can_purchase_storage_for_namespace?(namespace) && alert_level == :error
+- purchase_link = purchase_storage_url if can_purchase_storage_for_namespace?(namespace)
 - usage_link = namespace_storage_usage_link(root_namespace)
 - show_storage_banner_actions = purchase_link || usage_link
 
@@ -31,4 +31,4 @@
         - if purchase_link
           = link_to(_('Purchase more storage'), purchase_link, class: "btn gl-alert-action btn-md gl-button btn-success")
         - if usage_link
-          = link_to(_('Manage storage usage'), usage_link, class: "btn gl-alert-action btn-md gl-button btn-#{style}")
+          = link_to(_('Manage storage usage'), usage_link, class: "btn gl-alert-action btn-md gl-button btn-default")