prevent caching of ajax responses in the control panel

GET requests might be cached. Definitely happens on Internet Explorer. Makes it look like the user is getting unauthorized access.

See https://discourse.mailinabox.email/t/fresh-install-can-login-to-webmail-but-not-admin/394/4.

management/templates/index.html

@@ -329,6 +329,7 @@ function api(url, method, data, callback, callback_error) {
   ajax({
     url: "/admin" + url,
     method: method,
+    cache: false,
     data: data,
     beforeSend: function(xhr) {
       // We don't store user credentials in a cookie to avoid the hassle of CSRF