Fix a fatal when navigating to a Workboard after removing the Workboard menu item

Summary:
See PHI1247. If you remove the Workboard from a project profile menu, then navigate to the URI, we currently fatal when trying to select the "Workboard" item.

Instead, only try to select the item if some matching item is present.

Test Plan:

• Disabled the workboard on a project, navigated to /board/, got a sensible page with no navigation item selected instead of a fatal.
• Viewed a normal workboard, saw the correct selection.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20516