Add (admin-only) tickets via 'visibile' attribute [migration]

(cherry picked from commit ce0db3d7a736e7bcde480721986dbfb4a82aa083)

app/views/admin/tickets/index.html.haml

@@ -15,6 +15,7 @@
           %th Sold
           %th Turnover
           %th Registration Ticket
+          %th Visible?
           %th Actions
         %tbody
           - @conference.tickets.each do |ticket|
@@ -30,6 +31,8 @@
                 = humanized_money_with_symbol ticket.tickets_turnover_total(ticket.id)
               %td
                 = ticket.registration_ticket? ? 'Yes' : 'No'
+              %td
+                = ticket.visible? ? 'Yes' : 'No'
               %td
                 .btn-group
                   = link_to 'Edit', edit_admin_conference_ticket_path(@conference.short_title, ticket.id),