Fix `<x-data-provider>` demo helper element to load correctly [skip ci]
Fix `<x-data-provider>` demo helper element to load correctly [skip ci]

Changes applied:
  - Remove unnecessary wrapper `WebComponentsReady` event listener in
  the element definition
  - Remove unnecessary wrapper `<dom-module>` and merge script tags
  - Fix `users` and `getJSON` leaking to global scope

vaadin/components,Peppe/vaadin-grid,Peppe/vaadin-grid,vaadin/vaadin-grid,vaadin/components,vaadin/vaadin-grid,vaadin/components