Fix sub-second timing comparison error for Devise confirmation period

On databases such as MySQL, it's possible to get into a timing comparison
error if the value of `Confirmable#confirmation_sent_at` is within a second
of `0.days.ago`. This is possible mostly in specs that test this behavior
and most likely not happening in practice. The result of this error causes
a user to be deemed active when it should be inactive. To prevent this
error, we explicitly check the configuration setting to be `0.days.ago`.

Closes #2362

app/models/user.rb

@@ -1019,6 +1019,15 @@ class User < ActiveRecord::Base
     devise_mailer.send(notification, self, *args).deliver_later
   end
 
+  # This works around a bug in Devise 4.2.0 that erroneously causes a user to
+  # be considered active in MySQL specs due to a sub-second comparison
+  # issue. For more details, see: https://gitlab.com/gitlab-org/gitlab-ee/issues/2362#note_29004709
+  def confirmation_period_valid?
+    return false if self.class.allow_unconfirmed_access_for == 0.days
+
+    super
+  end
+
   def ensure_external_user_rights
     return unless external?