Feature: Send message

Implement and test out add message functionality. Basic use case would be:

1. Logged user, with valid jwt token sends POST http request to /<:u_id> endpoint, wishing to send message to user with id u_id
2. If logged user follows the user with id=u_id , add Message entity to database, otherwise return 403 (doesn't follow) or 404 (u_id user does not exist in database).