Allow installs to configure an arbitrary block of HTML to show on the login screen

Summary: For secure.phabricator.com, I've wanted to put up a "you can use demo/demo" message for a while. Wikimedia also wants to add some custom login instructions.

Test Plan:
Configured this:

    'auth.login-message' =>
      '<div style="width: 50%; margin: 1em auto; padding: 1em; border: 1px solid green; background: #dfffdf;">'.
        '<strong>Welcome!</strong> Lorem ipsum...'.
      '</div>',

...got this:

{F17167}

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran, jeremyb

Maniphest Tasks: T1637

Differential Revision: https://secure.phabricator.com/D3288

conf/default.conf.php

@@ -554,6 +554,11 @@ return array(
   // but rejects "joe@mail.yourcompany.com".
   'auth.email-domains' => array(),
 
+  // 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.
+  'auth.login-message' => null,
+
 
 // -- Accounts -------------------------------------------------------------- //
 

src/applications/auth/controller/PhabricatorLoginController.php

@@ -283,9 +283,12 @@ final class PhabricatorLoginController
       $panel->appendChild('<br />');
     }
 
+    $login_message = PhabricatorEnv::getEnvConfig('auth.login-message');
+
     return $this->buildStandardPageResponse(
       array(
         $error_view,
+        $login_message,
         $panel,
       ),
       array(