Simplify lookup of parent ExtJs panel

The Channel Manager app lives in an iframe, which lives in an ExtJs panel in the top-level DOM. The Channel Manager accesses this ExtJs panel via window.parent.Ext.getCmp('id-of-panel'). The ID is currently passed to the app via query parameter in the URL of the iframe. That's unnecessarily complex, since the ID is always the same. The lookup in the query parameter also requires mocking it in unit tests, which introduces even more complexity.

The lookup should be replaced with a constant.