(CE-819) Fix user login on confirming email after signup

For CloseMyAccount we need to make use of the standard login method
so we can use of the successful login hook, but when the user has
only just signed up and hasn't yet confirmed their email address, the
standard method fails due to the check for
UserLoginSpecialController::NOT_CONFIRMED_SIGNUP_OPTION_NAME. So, for
the email confirmation on initial signup, log the user in manually as
was done before commit 99ed1d1b0, as we don't need to hook in if
they've never logged in before as is the case for users just after
signup.