Logout token management

[Eraldo]

I suggest also adding a method that takes care of device and subscription handling when a user logs out of the device.
Otherwise the device will keep getting notifications!

This might entail:

• Invalidating active tokens. (active=False)
• Unsubscribing the token from all topics.

[xtrinch]

In relation to my comment on #264, what could be done is on device deletion, the token unsubscribed from all topics