Add ability to set 'admin' custom claim

[alaksej]

https://firebase.google.com/docs/auth/admin/custom-claims#defining_roles_via_backend_script

• add storage rule

  service firebase.storage {
  match /b/{bucket}/o {
  match /files/{authGroupName}/{fileName} {
    allow read: if request.auth.token.admin == true
  }
  }
  }

[alaksej]

added via the cloud functions