Allow dependent fields to work with multiple dependencies

Previously we were clobbering any query params that had been set by a
previous update to dependent fields. This PR makes it so that we retain
any query params that were already set and we just update the one that
has changed.

bullet_train-fields/app/javascript/controllers/dependent_fields_frame_controller.js

@@ -32,7 +32,7 @@ export default class extends Controller {
   }
 
   constructNewUrlUpdatingField(fieldName, fieldValue) {
-    const url = new URL(window.location.href)
+    const url = new URL(this.element.src || window.location.href)
     url.searchParams.set(fieldName, fieldValue)
 
     return url.href