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/PhabricatorProjectEditController.php

@@ -24,6 +24,11 @@ final class PhabricatorProjectEditController
 
     $id = $request->getURIData('id');
     if (!$id) {
+      // This capability is checked again later, but checking it here
+      // explicitly gives us a better error message.
+      $this->requireApplicationCapability(
+        ProjectCreateProjectsCapability::CAPABILITY);
+
       $parent_id = head($request->getArr('parent'));
       if (!$parent_id) {
         $parent_id = $request->getStr('parent');