Setup notification config

[rossicler]

Issue Description

Setup a notification config that allow an API to send an app notification to a specific user
If needed, add a new column on the User model to store the user device id (or something like that)
The notification can be sent to a specific User (using user.id or some other unique key) or a group of user (using an array of unique keys) or to all registered users.

Tasks:

• Make the relation of an User to a device
• Setup a way to send a notification through the relation between a device and a user