Improve subproject/milestone error handling for users who can't create projects

Summary:
Fixes T10357.

  - Show a better (more descriptive) error message when a user who can't create projects tries to create a subproject or milestone.
  - Disable the subproject actions if you don't have create permission.

All this stuff was already enforced properly: this diff doesn't make any actual policy changes, just improves the UI for users who lack permission.

Test Plan:
  - As an unprivileged user (no "Can Create Projects"), tried to create a subproject or milestone.
  - After patch, got a disabled action, with more specific and helpful error than before.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10357

Differential Revision: https://secure.phabricator.com/D15274

src/applications/project/controller/PhabricatorProjectSubprojectsController.php

@@ -181,6 +181,9 @@ final class PhabricatorProjectSubprojectsController
     $viewer = $this->getViewer();
     $id = $project->getID();
 
+    $can_create = $this->hasApplicationCapability(
+      ProjectCreateProjectsCapability::CAPABILITY);
+
     $can_edit = PhabricatorPolicyFilter::hasCapability(
       $viewer,
       $project,
@@ -198,7 +201,7 @@ final class PhabricatorProjectSubprojectsController
       $milestone_text = pht('Create Milestone');
     }
 
-    $can_milestone = ($can_edit && $allows_milestones);
+    $can_milestone = ($can_create && $can_edit && $allows_milestones);
     $milestone_href = "/project/edit/?milestone={$id}";
 
     $view->addAction(
@@ -209,7 +212,7 @@ final class PhabricatorProjectSubprojectsController
         ->setDisabled(!$can_milestone)
         ->setWorkflow(!$can_milestone));
 
-    $can_subproject = ($can_edit && $allows_subprojects);
+    $can_subproject = ($can_create && $can_edit && $allows_subprojects);
 
     // If we're offering to create the first subproject, we're going to warn
     // the user about the effects before moving forward.