Push service endpoints

push-endpoints.txt

POST /push - subscribe user by user_id with device_token
param: device_token: String
param: user_id: Int
Authorization: token: String - user Auth token

returns: 
{
  ...
  data: {
    user_id: Int,
    subscription_id: Int
  }
}

DELETE /push/:subscription_id - unsubscribe user by subscription_id
Authorization: token: String - user Auth token

{
  ...
  message: String
}

db model:
PushSubscription {
  subscription_id: Int (pk, autonumber)
  user_id: String
  deviceToken: String
}

// multiple users can have multiple devices registered
// on user logout user subscription is deleted for specific device