save the push token of new device even if push notifications are not
enabled and provide a way to register the push device at login
unregister device if there already is a push token saved unless the
new token has already been registered.
also the `unregister_push_device` function used the wrong argument
cf. 08d380900b/src/Core/Services/Implementations/RelayPushRegistrationService.cs (L43)
Stefan Melmuk
THONY
tuhana
Bernd Schoolmann
GeekCornerGH