Handling users through the DB `users` table

[vporton]

I changed the DB schema to store users in DB: users table created.

I use now user_ref (int) instead of user_id (VARCHAR) to refer to a user.

In UserInputDto I use pk to refer to the user instead of, as before, his/her email.

I did basic testing and it appears to work, more testing is however needed.

Fixes #6.