Registration of existing email should maybe always require a confirmation

[Kauhsa]

Currently, server doesn't seem to ask confirmation if "device ID" matches (see Utils.getDeviceId() method on Android client), but this is probably insecure since the device id is not guaranteed to be unique.

Need to figure out a better solution.

[ffagerho]

Related to #65.

[anzhieta]

Fixing this right now would cause the server to require confirmation every time you sign in, since signing out clears your existing authentication token. A proper fix would be to use passwords to authenticate sign ins, but that requires major changes in both the app and the server