Fix bug.

Cannot add a `let` variable to an object.

app/assets/javascripts/qpixel_api.js

 const validators = [];
 
+/** Counts notifications popped up at any time. */
+let popped_modals_ct = 0;
+
 window.QPixel = {
   /**
    * Get the current CSRF anti-forgery token. Should be passed as the X-CSRF-Token header when
@@ -12,9 +15,6 @@ window.QPixel = {
     return token;
   },
 
-  /** Counts notifications popped up at any time. */
-  let popped_modals_ct = 0;
-
   /**
    * Create a notification popup - not an inbox notification.
    * @param type the type to apply to the popup - warning, danger, etc.