When we try to register with omniauth and the email or username already exists,
we use the finish_signup and do_finish_signup actions to allow the user to choose
another email or username.
The do_finish_signup action of the registration controller calls the
send_oauth_confirmation_instructions method which is responsible for sending the
confirmation email.
In this method we were only validating the case that the email is duplicated. Now
we add one more condition that allows us to send the instructions for the case in
which we have had to change our username.
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