The Greeter should not allow enabling read-write persistence on read-only media

It has been reported that:

on boot, if persistence is enabled but read-only is not selected, the system hangs on read-only usb sticks

Perhaps read-only should be automatically selected on such media, or at least it should show an error message.

Parent Task: #7019

Original created by @alant on 6611 (Redmine)