Show account cards for external accounts on "linked accounts" screen and "link new account"

Summary: Ref T1536. These can probably use some design tweaking and there's a bit of a bug with profile images for some providers, but generally seems to be in the right ballpark.

Test Plan:
{F46604}

{F46605}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6210

src/applications/auth/controller/PhabricatorAuthConfirmLinkController.php

@@ -60,7 +60,12 @@ final class PhabricatorAuthConfirmLinkController
           pht(
             "Confirm the link with this %s account. This account will be ".
             "able to log in to your Phabricator account.",
-            $provider->getProviderName())));
+            $provider->getProviderName())))
+      ->appendChild(
+        id(new PhabricatorAuthAccountView())
+          ->setUser($viewer)
+          ->setExternalAccount($account)
+          ->setAuthProvider($provider));
 
     $dialog->appendChild($form);