Make "profile menu" configuration mostly work

Summary:
Ref T10054. This does a big chunk of the legwork to let users reconfigure profile menus (currently, just project menus).

This includes:

  - Editing builtin items (e.g., you can rename the default items).
  - Creating new items (for now, only links are available).

This does not yet include:

  - Hiding items.
  - Reordering items.
  - Lots of fancy types of items (dashboards, etc).
  - Any UI changes.
  - Documentation (does feature: TODO link for documentation).

Test Plan:
{F1060695}

{F1060696}

{F1060697}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10054

Differential Revision: https://secure.phabricator.com/D15010

src/applications/transactions/controller/PhabricatorEditEngineConfigurationListController.php

@@ -16,6 +16,10 @@ final class PhabricatorEditEngineConfigurationListController
     $engine = PhabricatorEditEngine::getByKey($viewer, $engine_key)
       ->setViewer($viewer);
 
+    if (!$engine->isEngineConfigurable()) {
+      return new Aphront404Response();
+    }
+
     $items = array();
     $items[] = id(new PHUIListItemView())
       ->setType(PHUIListItemView::TYPE_LABEL)