Add new config.json setting to allow to specify staging sites for testing purposes

This is to avoid having to create multi-site apps publicly displaying staging sites

First of all, this can be enabled only via config.json, "stagingsites": (if the array is empty or not set or false we should not display the toggle in Development settings)

To view the list of staging sites the tester will have to enable a special option in the app Development settings "Enable staging sites" (toggle)

Once enabled, the app should display on the first screen the list of sites (simulating what a multi-site app would do).