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

(cherry picked from commit ce0db3d7a736e7bcde480721986dbfb4a82aa083)

app/models/ability.rb

@@ -80,7 +80,9 @@ class Ability
     end
 
     can :index, Organization
-    can :index, Ticket
+    can :index, Ticket do |ticket|
+      ticket.visible
+    end
     can :manage, TicketPurchase, user_id: user.id
     can [:new, :create], Payment, user_id: user.id
     can [:index, :show], PhysicalTicket, user: user

app/models/ticket.rb

@@ -18,6 +18,8 @@ class Ticket < ApplicationRecord
 
   validates :price_cents, numericality: { greater_than_or_equal_to: 0 }
 
+  scope :visible, -> { where(visible: true) }
+
   def bought?(user)
     buyers.include?(user)
   end

app/models/ticket_purchase.rb

@@ -32,7 +32,7 @@ class TicketPurchase < ApplicationRecord
       errors.push('You cannot buy more than one registration tickets.')
     else
       ActiveRecord::Base.transaction do
-        conference.tickets.each do |ticket|
+        conference.tickets.visible.each do |ticket|
           quantity = purchases[ticket.id.to_s].to_i
           # if the user bought the ticket and is still unpaid, just update the quantity
           purchase = if ticket.bought?(user) && ticket.unpaid?(user)