Fix user already subscribed exception

Prevent excpetion at user subscription when already is subscribed to a conference and add hanling to the similar unsubscribe event.

remove double validation at app/models/subscription.rb

update subscription_controller_spec

app/models/subscription.rb

@@ -1,5 +1,4 @@
 class Subscription < ActiveRecord::Base
-  validates :user_id, uniqueness: { scope: [:conference_id] }
   belongs_to :conference
   belongs_to :user