RegisterType registers the type used in the user handlers and the corresponding query for retrieving it from the database.

ErrEmptyUserPass happens when no username and/or password is given for a user.

ErrInvalidPassword is returned when an empty password is given for login or signup.

ErrInvalidUserName is returned when an empty username is given for login or signup.

ErrInvalidUserPass happens when no valid username and/or password is given for a user.

User represents a user of the app.

Interface must be satisfied by the registered user type.