On portals, make the "selected" / "default" logic more straightforward

Summary:
Depends on D20349. Ref T13275. Currently, a default item is selected as a side effect of generating the full list of items, for absolutely no reason.

The logic to pick the currently selected item can also be separated out pretty easily.

(And fix a bug in with a weird edge case in projects.)

This doesn't really change anything, but it will probably make T12949 a bit easier to fix.

Test Plan: Viewed Home / projects / portals, clicked various links, got same default/selection behavior as before.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13275

Differential Revision: https://secure.phabricator.com/D20352

src/applications/project/controller/PhabricatorProjectViewController.php

@@ -28,7 +28,7 @@ final class PhabricatorProjectViewController
       $default_key = PhabricatorProject::ITEM_MANAGE;
     }
 
-    switch ($default->getBuiltinKey()) {
+    switch ($default_key) {
       case PhabricatorProject::ITEM_WORKBOARD:
         $controller_object = new PhabricatorProjectBoardViewController();
         break;