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

test/javascripts/acceptance/plugin-outlets-test.js.es6

@@ -1,16 +0,0 @@
-import { acceptance } from "helpers/qunit-helpers";
-
-acceptance("Discourse Subscriptions", {
-  settings: {
-    discourse_subscriptions_extra_nav_subscribe: true
-  }
-});
-
-QUnit.test("plugin outlets", async assert => {
-  await visit("/");
-
-  assert.ok(
-    $("#navigation-bar .discourse-patrons-subscribe").length,
-    "has a subscribe button"
-  );
-});