Add a blanket "will login" event

Summary:
Ref T1536. Facebook currently does a check which should be on-login in registration hooks, and this is generally a reasonable hook to provide.

The "will login" event allows listeners to reject or modify a login, or just log it or whatever.

NOTE: This doesn't cover non-web logins right now -- notably Conduit. That's presumably fine.

(This can't land for a while, it depends on about 10 uncommitted revisions.)

Test Plan: Logged out and in again.

Reviewers: wez, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6202

src/applications/auth/controller/PhabricatorAuthRegisterController.php

@@ -205,13 +205,11 @@ final class PhabricatorAuthRegisterController
 
           $user->saveTransaction();
 
-          $this->establishWebSession($user);
-
           if (!$email_obj->getIsVerified()) {
             $email_obj->sendVerificationEmail($user);
           }
 
-          return $this->buildLoginValidateResponse($user);
+          return $this->loginUser($user);
         } catch (AphrontQueryDuplicateKeyException $exception) {
           $same_username = id(new PhabricatorUser())->loadOneWhere(
             'userName = %s',
@@ -480,7 +478,7 @@ final class PhabricatorAuthRegisterController
     }
   }
 
-  private function renderError($message) {
+  protected function renderError($message) {
     return $this->renderErrorPage(
       pht('Registration Failed'),
       array($message));