REFACTOR: Use models to store data (#11)

* REFACTOR: Use api to add subscribe link

* FIX: I18n subscribe link

* REFACTOR: Use models to store some data

This enables the plugin to show only subscription information which was
generated on Discourse. Subscription data storage is limited to the
external identifiers Stripe generates so we can interact with the API.

* DEV: Test/linting fixes/rake task

app/models/customer.rb

@@ -6,6 +6,8 @@ module DiscourseSubscriptions
 
     scope :find_user, ->(user) { find_by_user_id(user.id) }
 
+    has_many :subscriptions
+
     def self.create_customer(user, customer)
       create(customer_id: customer[:id], user_id: user.id)
     end

app/models/product.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+module DiscourseSubscriptions
+  class Product < ActiveRecord::Base
+  end
+end

app/models/subscription.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+module DiscourseSubscriptions
+  class Subscription < ActiveRecord::Base
+    belongs_to :customer
+  end
+end