Remove all code referencing old tab navigation

Summary:
  - Add getHelpURI() to PhabricatorApplication for application user guides.
  - Add a new "help" icon menu item and skeletal Diviner application.
  - Move help tabs to Applications where they exist, document the other ones that don't exist yet.
  - Grep for all tab-related stuff and delete it.

Test Plan: Clicked "help" for some apps. Clicked around randomly in a bunch of other apps.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3267

src/applications/differential/controller/DifferentialController.php

@@ -34,7 +34,6 @@ abstract class DifferentialController extends PhabricatorController {
     $page->setTitle(idx($data, 'title'));
     $page->setGlyph("\xE2\x9A\x99");
     $page->appendChild($view);
-    $page->setIsLoggedOut($viewer_is_anonymous);
     $page->setSearchDefaultScope(PhabricatorSearchScope::SCOPE_OPEN_REVISIONS);
 
     $response = new AphrontWebpageResponse();