When no master database is configured, automatically degrade to read-only mode

Summary: Ref T4571. If `cluster.databases` is configured but only has replicas, implicitly drop to read-only mode and send writes to a replica.

Test Plan:
  - Disabled the `master`, saw Phabricator automatically degrade into read-only mode against replicas.
  - (Also tested: explicit read-only mode, non-cluster mode, properly configured cluster mode).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4571

Differential Revision: https://secure.phabricator.com/D15672

src/applications/system/controller/PhabricatorSystemReadOnlyController.php

@@ -25,6 +25,27 @@ final class PhabricatorSystemReadOnlyController
           'has been turned on by rolling your chair away from your desk and '.
           'yelling "Hey! Why is Phabricator in read-only mode??!" using '.
           'your very loudest outside voice.');
+        $body[] = pht(
+          'This mode is active because it is enabled in the configuration '.
+          'option "%s".',
+          phutil_tag('tt', array(), 'cluster.read-only'));
+        $button = pht('Wait Patiently');
+        break;
+      case PhabricatorEnv::READONLY_MASTERLESS:
+        $title = pht('No Writable Database');
+        $body[] = pht(
+          'Phabricator is currently configured with no writable ("master") '.
+          'database, so it can not write new information anywhere. '.
+          'Phabricator will run in read-only mode until an administrator '.
+          'reconfigures it with a writable database.');
+        $body[] = pht(
+          'This usually occurs when an administrator is actively working on '.
+          'fixing a temporary configuration or deployment problem.');
+        $body[] = pht(
+          'This mode is active because no database has a "%s" role in '.
+          'the configuration option "%s".',
+          phutil_tag('tt', array(), 'master'),
+          phutil_tag('tt', array(), 'cluster.databases'));
         $button = pht('Wait Patiently');
         break;
       default:
@@ -33,8 +54,8 @@ final class PhabricatorSystemReadOnlyController
 
     $body[] = pht(
       'In read-only mode you can read existing information, but you will not '.
-      'be able to edit information or create new information until this mode '.
-      'is disabled.');
+      'be able to edit objects or create new objects until this mode is '.
+      'disabled.');
 
     $dialog = $this->newDialog()
       ->setTitle($title)