SAAS-973: Airtime Billing page - Add support for August promotion plans

Fix to not allow current promo plans to downgrade to another promo plan
Added promo banner to billing page

airtime_mvc/application/views/scripts/billing/upgrade.phtml

@@ -196,6 +196,10 @@ $(document).ready(function() {
 <div class="ui-widget ui-widget-content block-shadow clearfix padded-strong billing-panel">
 <H2><?=_("Account Plans")?></H2>
 <H4><?=_("Upgrade today to get more listeners and storage space!")?></H4>
+<div>
+        <a href="https://www.airtime.pro/pricing#promo-details" target="_blank">
+            <img width="400px" height="133px" class="promo-banner" /></a>
+    </div>
 <div class="pricing-grid">
     <table>
         <tr>