feat(promotion code): Promotion code system done

I added the features for users to use promotion code
and for promoters to create on their events.
May be rewrite to discount code?

app/models/event.rb

@@ -21,6 +21,7 @@ class Event < ApplicationRecord
   has_many :ticket_types
   has_many :tickets, through: :ticket_types
   has_many :orders
+  has_many :promotion_codes
 
   # === Callbacks ===
   before_validation :geocode_address, if: :should_geocode_address?