Add box-shadow focus effect to Dashboard's "Manage Panel" button

Summary: Apply that strange blue `box-shadow` CSS on `button:focus` just like everywhere else.

Test Plan: * Go to http://phorge.localhost/dashboard/view/1/ which already has a panel added, and click (but don't release) the "Manage Panel" button. It should have a blue glow just like any other "Cancel" button or "Configure Form" on http://phorge.localhost/transactions/editengine/maniphest.task/edit/1/

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D26397

src/applications/dashboard/engine/PhabricatorDashboardPanelRenderingEngine.php

···
     $action_menu = id(new PHUIButtonView())
       ->setTag('a')
       ->setIcon('fa-cog')
+      ->setHref('#')
       ->setText(pht('Manage Panel'))
       ->setDropdownMenu($dropdown_menu);