add stripe integration for safebooru

app/helpers/user_upgrades_helper.rb

@@ -0,0 +1,20 @@
+module UserUpgradesHelper
+  def stripe_button(desc, cost)
+    html = %{
+      <form action="#{user_upgrade_path}" method="POST" class="stripe">
+        <input type="hidden" name="authenticity_token" value="#{form_authenticity_token}">
+        #{hidden_field_tag(:desc, desc)}
+        <script
+          src="https://checkout.stripe.com/checkout.js" class="stripe-button"
+          data-key="#{Danbooru.config.stripe_publishable_key}"
+          data-name="#{Danbooru.config.app_name}"
+          data-description="#{desc}"
+          data-label="#{desc}"
+          data-amount="#{cost}">
+        </script>
+      </form>
+    }
+
+    raw(html)
+  end
+end