[App] Guest / read-only access for the app

To let others (then the owner) (=guests) use the app and don’t give full access to the settings as well, it should be possible for those to get an read-only account.
So those can set up the app individually according to units and alerts. Bot not being able to change station specific settings.

In addition the accounts (those of owners as well) will become able to “subscribe” to stations of different owners.
And finally, be able to set the alarms individual for each subscribed and owned station.