Fix a fatal in the "Projects" curtain extension when a project edge connects an object to a non-project

Summary:
Ref T13429. It's currently possible to write "TYPE_EDGE" relationships for the "object has project" edge to PHIDs which may not actually be projects. Today, this fatals.

As a first step, unfatal it. T13429 discusses general improvements and greater context.

Test Plan:
Used "maniphest.edit" to write a "project" edge to a user PHID, viewed the task in the UI. Previously it fataled; now it renders unusually (the object is "tagged" with a user) but faithfully reflects database state.

{F6957606}

Maniphest Tasks: T13429

Differential Revision: https://secure.phabricator.com/D20860

src/__phutil_library_map__.php

@@ -5044,6 +5044,7 @@ phutil_register_library_map(array(
     'PhabricatorWeekStartDaySetting' => 'applications/settings/setting/PhabricatorWeekStartDaySetting.php',
     'PhabricatorWildConfigType' => 'applications/config/type/PhabricatorWildConfigType.php',
     'PhabricatorWordPressAuthProvider' => 'applications/auth/provider/PhabricatorWordPressAuthProvider.php',
+    'PhabricatorWorkboardInterface' => 'applications/project/interface/PhabricatorWorkboardInterface.php',
     'PhabricatorWorkboardViewState' => 'applications/project/state/PhabricatorWorkboardViewState.php',
     'PhabricatorWorker' => 'infrastructure/daemon/workers/PhabricatorWorker.php',
     'PhabricatorWorkerActiveTask' => 'infrastructure/daemon/workers/storage/PhabricatorWorkerActiveTask.php',
@@ -10748,6 +10749,7 @@ phutil_register_library_map(array(
       'PhabricatorColumnProxyInterface',
       'PhabricatorSpacesInterface',
       'PhabricatorEditEngineSubtypeInterface',
+      'PhabricatorWorkboardInterface',
     ),
     'PhabricatorProjectActivityChartEngine' => 'PhabricatorChartEngine',
     'PhabricatorProjectAddHeraldAction' => 'PhabricatorProjectHeraldAction',

src/applications/project/engine/PhabricatorBoardLayoutEngine.php

@@ -301,6 +301,12 @@ final class PhabricatorBoardLayoutEngine extends Phobject {
       ->execute();
     $boards = mpull($boards, null, 'getPHID');
 
+    foreach ($boards as $key => $board) {
+      if (!($board instanceof PhabricatorWorkboardInterface)) {
+        unset($boards[$key]);
+      }
+    }
+
     if (!$this->fetchAllBoards) {
       foreach ($boards as $key => $board) {
         if (!$board->getHasWorkboard()) {

src/applications/project/interface/PhabricatorWorkboardInterface.php

@@ -0,0 +1,3 @@
+<?php
+
+interface PhabricatorWorkboardInterface {}

src/applications/project/storage/PhabricatorProject.php

@@ -13,7 +13,8 @@ final class PhabricatorProject extends PhabricatorProjectDAO
     PhabricatorConduitResultInterface,
     PhabricatorColumnProxyInterface,
     PhabricatorSpacesInterface,
-    PhabricatorEditEngineSubtypeInterface {
+    PhabricatorEditEngineSubtypeInterface,
+    PhabricatorWorkboardInterface {
 
   protected $name;
   protected $status = PhabricatorProjectStatus::STATUS_ACTIVE;