Enable notifications via settings api

[IonesioJunior]

Using the new Notifier Service, the node admin must be able to enable/disable the node notifier.

But to achieve that, there are some edge cases to keep in mind.

• By UX/UI reasons, the api used to enable / disable it will be set at client.settints.
  • To enable: client.settings.enable_notifications(token=<TOKEN>)
  • To disable: client.settings.disable_notifications()
• If email token isn't provided, returns a SyftError informing that email_token parameter is mandatory.
• If a regular user tries to use this API, it must receive the regular PySyft non authorized error.
• If admin provides all the informations properly, it will return a SyftSuccess message.

[jcardonnet]

LGTM