Add an option to remove all subscriptions for a user

[TysonAndre]

Reason: this is slightly more convenient than fetching subscriptions and deleting them individually.

Add a new option alldevices=1 to delete any subscriptions from the service (or comma separated service list?)

/unsubscribe?service=<servicename>&subscriber=uniqush.client1234&alldevices=1

Example use case: A user of an app has deleted their account (which affects all devices), so the backend (using uniqush) would free up the memory used for the subscription and prevent new pushes.

This should report success if all device tokens or regids were deleted (and services/psps were fetched successfully), even if there was nothing to delete