Modularize generation of supplemental login messages

Summary:
Ref T9346. This mostly allows us to give users additional advice based on which instance they are trying to log in to in the Phacility cluster.

It's also slightly more flexible than `auth.login-message` was, and maybe we'll add some more hooks here eventually.

This feels like it's a sidegrade in complexity rather than really an improvement, but not too terrible.

Test Plan:
  - Wrote the custom handler in T9346 to replicate old config functionality.
  - Wrote a smart handler for Phacility that can provide context-sensitive messages based on which OAuth client you're trying to use.

See new message box at top (implementation in next diff):

{F780375}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9346

Differential Revision: https://secure.phabricator.com/D14057

src/__phutil_library_map__.php

@@ -1613,6 +1613,7 @@ phutil_register_library_map(array(
     'PhabricatorAuthLinkController' => 'applications/auth/controller/PhabricatorAuthLinkController.php',
     'PhabricatorAuthListController' => 'applications/auth/controller/config/PhabricatorAuthListController.php',
     'PhabricatorAuthLoginController' => 'applications/auth/controller/PhabricatorAuthLoginController.php',
+    'PhabricatorAuthLoginHandler' => 'applications/auth/handler/PhabricatorAuthLoginHandler.php',
     'PhabricatorAuthManagementCachePKCS8Workflow' => 'applications/auth/management/PhabricatorAuthManagementCachePKCS8Workflow.php',
     'PhabricatorAuthManagementLDAPWorkflow' => 'applications/auth/management/PhabricatorAuthManagementLDAPWorkflow.php',
     'PhabricatorAuthManagementListFactorsWorkflow' => 'applications/auth/management/PhabricatorAuthManagementListFactorsWorkflow.php',
@@ -5458,6 +5459,7 @@ phutil_register_library_map(array(
     'PhabricatorAuthLinkController' => 'PhabricatorAuthController',
     'PhabricatorAuthListController' => 'PhabricatorAuthProviderConfigController',
     'PhabricatorAuthLoginController' => 'PhabricatorAuthController',
+    'PhabricatorAuthLoginHandler' => 'Phobject',
     'PhabricatorAuthManagementCachePKCS8Workflow' => 'PhabricatorAuthManagementWorkflow',
     'PhabricatorAuthManagementLDAPWorkflow' => 'PhabricatorAuthManagementWorkflow',
     'PhabricatorAuthManagementListFactorsWorkflow' => 'PhabricatorAuthManagementWorkflow',

src/applications/auth/controller/PhabricatorAuthStartController.php

@@ -163,8 +163,22 @@ final class PhabricatorAuthStartController
         $button_columns);
     }
 
-    $login_message = PhabricatorEnv::getEnvConfig('auth.login-message');
-    $login_message = phutil_safe_html($login_message);
+    $handlers = PhabricatorAuthLoginHandler::getAllHandlers();
+
+    $delegating_controller = $this->getDelegatingController();
+
+    $header = array();
+    foreach ($handlers as $handler) {
+      $handler = clone $handler;
+
+      $handler->setRequest($request);
+
+      if ($delegating_controller) {
+        $handler->setDelegatingController($delegating_controller);
+      }
+
+      $header[] = $handler->getAuthLoginHeaderContent();
+    }
 
     $invite_message = null;
     if ($invite) {
@@ -178,7 +192,7 @@ final class PhabricatorAuthStartController
     return $this->buildApplicationPage(
       array(
         $crumbs,
-        $login_message,
+        $header,
         $invite_message,
         $out,
       ),

src/applications/auth/handler/PhabricatorAuthLoginHandler.php

@@ -0,0 +1,36 @@
+<?php
+
+abstract class PhabricatorAuthLoginHandler
+  extends Phobject {
+
+  private $request;
+  private $delegatingController;
+
+  public function getAuthLoginHeaderContent() {
+    return array();
+  }
+
+  final public function setDelegatingController(AphrontController $controller) {
+    $this->delegatingController = $controller;
+    return $this;
+  }
+
+  final public function getDelegatingController() {
+    return $this->delegatingController;
+  }
+
+  final public function setRequest(AphrontRequest $request) {
+    $this->request = $request;
+    return $this;
+  }
+
+  final public function getRequest() {
+    return $this->request;
+  }
+
+  final public static function getAllHandlers() {
+    return id(new PhutilClassMapQuery())
+      ->setAncestorClass(__CLASS__)
+      ->execute();
+  }
+}

src/applications/config/check/PhabricatorExtraConfigSetupCheck.php

@@ -276,6 +276,10 @@ final class PhabricatorExtraConfigSetupCheck extends PhabricatorSetupCheck {
         'Impersonating users over the API is no longer supported.'),
 
       'feed.public' => pht('The framable public feed is no longer supported.'),
+
+      'auth.login-message' => pht(
+        'This configuration option has been replaced with a modular '.
+        'handler. See T9346.'),
     );
 
     return $ancient_config;

src/applications/config/option/PhabricatorAuthenticationConfigOptions.php

@@ -73,14 +73,6 @@ final class PhabricatorAuthenticationConfigOptions
         ->addExample(
           "yourcompany.com\nmail.yourcompany.com",
           pht('Valid Setting')),
-      $this->newOption('auth.login-message', 'string', null)
-        ->setLocked(true)
-        ->setSummary(pht('A block of HTML displayed on the login screen.'))
-        ->setDescription(
-          pht(
-            "You can provide an arbitrary block of HTML here, which will ".
-            "appear on the login screen. Normally, you'd use this to provide ".
-            "login or registration instructions to users.")),
       $this->newOption('account.editable', 'bool', true)
         ->setBoolOptions(
           array(