Write ExternalAccountIdentifiers when interacting with external authentication providers

Summary:
Depends on D21015. When we sync an external account and get a list of account identifiers, write them to the database.

Nothing reads them yet and we still write "accountId", this just prepares us for reads.

Test Plan: Linked, refreshed, unlinked, and re-linked an external account. Peeked at the database and saw a sensible-looking row.

Differential Revision: https://secure.phabricator.com/D21016

src/applications/auth/controller/PhabricatorAuthUnlinkController.php

@@ -67,7 +67,7 @@ final class PhabricatorAuthUnlinkController
       ->setWorkflowKey($workflow_key)
       ->requireHighSecurityToken($viewer, $request, $done_uri);
 
-    $account->delete();
+    $account->unlinkAccount();
 
     id(new PhabricatorAuthSessionEngine())->terminateLoginSessions(
       $viewer,