added download certificate and key in zip format

api/frontend/js/certs.js

@@ -34,9 +34,9 @@ const renderCards = () => {
               >
                   Cert
               </button>
-              <button class="btn btn-primary" style="width: 100%">
+              <a class="btn btn-primary" style="width: 100%" href="${window.location.origin + "/api/certs/download/?domain=" + cert.certinfo.DNSNames[0]}" download="${cert.certinfo.DNSNames[0]}.zip">
                   Download
-              </button>
+              </a>
               <button class="btn btn-danger delcert" id="${cert.id}" style="width: 100%">
                   Delete
               </button>