FIX: Fix registration dialog popup for 'full screen' social logins

Regression following the ember3 upgrade. In addition to fixing, this commit consolidates our social registration logic into one place, and adds tests for the behaviour.

test/javascripts/acceptance/auth-complete-test.js.es6

@@ -0,0 +1,40 @@
+import { acceptance } from "helpers/qunit-helpers";
+acceptance("Auth Complete", {
+  beforeEach() {
+    const node = document.createElement("meta");
+    node.dataset.authenticationData = JSON.stringify({
+      auth_provider: "test",
+      email: "blah@example.com"
+    });
+    node.id = "data-authentication";
+    document.querySelector("head").appendChild(node);
+  },
+  afterEach() {
+    document
+      .querySelector("head")
+      .removeChild(document.getElementById("data-authentication"));
+  }
+});
+
+QUnit.test("when login not required", async assert => {
+  await visit("/");
+
+  assert.equal(currentPath(), "discovery.latest", "it stays on the homepage");
+
+  assert.ok(
+    exists("#discourse-modal div.create-account"),
+    "it shows the registration modal"
+  );
+});
+
+QUnit.test("when login required", async assert => {
+  Discourse.SiteSettings.login_required = true;
+  await visit("/");
+
+  assert.equal(currentPath(), "login", "it redirects to the login page");
+
+  assert.ok(
+    exists("#discourse-modal div.create-account"),
+    "it shows the registration modal"
+  );
+});