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

db/migrate/20191203014808_create_subscriptions_customers.rb

@@ -3,9 +3,9 @@
 class CreateSubscriptionsCustomers < ActiveRecord::Migration[6.0]
   def change
     create_table :discourse_subscriptions_customers do |t|
-      t.string :customer_id, null: false
+      t.string :customer_id, null: false, index: true
       t.string :product_id
-      t.references :user, foreign_key: true
+      t.references :user
       t.timestamps
     end
   end

db/migrate/20200514175537_create_products.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class CreateProducts < ActiveRecord::Migration[6.0]
+  def change
+    create_table :discourse_subscriptions_products do |t|
+      t.string :external_id, null: false, index: true
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20200518145424_create_subscriptions.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class CreateSubscriptions < ActiveRecord::Migration[6.0]
+  def change
+    create_table :discourse_subscriptions_subscriptions do |t|
+      t.integer :customer_id, null: false, index: true
+      t.string :external_id, null: false, index: true
+
+      t.timestamps
+    end
+  end
+end