Guest cannot submit Links for existing author_email

[tiagof]

Scenario

Given I'm a guest, when submitting a new Link, the system should return an HTTP error 422 when the provided author_email is already "in use" by a registered user, i.e., the value of author_email cannot exist in the email column of the users table.

Given I'm an authenticated user of any type, when submitting a new Link, the system should ignore the provided author_name and author_email and assign, respectively, my name and email to these fields.