Add a dialog explaining what to do next when tapping "Verify this device"

The menu that has this action is now more discoverable, and this menu item is plagued with a bad UX - when you tap on it nothing obvious happens! To people not familiar with device verification they will think this is a bug, but in fact all they need to do is open another verified session on another device.

So now there's a dialog explaining that the next step is to do just that. This dialog also closes once the verification session starts, but the user has the option to close it in the meantime.