Show name and email for deleted poll officer's user account

Avoid to raise an exception `Module::DelegationError' when trying to
show the name and/or email of a poll officer whose user account has
been deleted.
We'll show a message "User deleted" and "Email deleted" instead.

app/models/poll/officer.rb

@@ -7,7 +7,13 @@ class Poll
 
     validates :user_id, presence: true, uniqueness: true
 
-    delegate :name, :email, to: :user
+    def name
+      user&.name || I18n.t("shared.author_info.author_deleted")
+    end
+
+    def email
+      user&.email || I18n.t("shared.author_info.email_deleted")
+    end
 
     def voting_days_assigned_polls
       officer_assignments.voting_days.includes(booth_assignment: :poll).