move subscribe to its own controller, add authorization, test subscribe/unsubscribe

app/models/ability.rb

@@ -129,6 +129,9 @@ class Ability
   def signed_in(user)
     guest # Inherits abilities of guest
 
+    # Can subscribe, unsubscribe to a conference
+    can [:create, :destroy], Subscription, user_id: user.id
+
     # Conference Registration
     can :manage, Registration, user_id: user.id
 

app/models/user.rb

@@ -55,6 +55,10 @@ class User < ActiveRecord::Base
             },
             presence: true
 
+  def subscribed? conference
+    self.subscriptions.find_by(conference_id: conference.id).present?
+  end
+
   # Returns the ticket purchased ticket
   # ====Returns
   # * +TicketUser::ActiveRecord_Relation+ -> user