Fix UX regression in Dashboard Tabs Panel

Summary:
Before this change, it seems that the Dashboard Tabs Panel could be empty
as default. This was the regression:

cbc0e66154

The variable $is_selected was defined inside a foreach loop, but that loop
was terminated when it was then read again. This redefines that variable in
the right scope.

| Before    | After     |
|-----------|-----------|
| {F312797} | {F312798} |

Closes T15474

Test Plan: - see that now you have something visibile in your Dashboard Tabs Panel

Reviewers: speck, O1 Blessed Committers

Reviewed By: speck, O1 Blessed Committers

Subscribers: tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15474, T15146

Differential Revision: https://we.phorge.it/D25291

src/applications/dashboard/paneltype/PhabricatorDashboardTabsPanelType.php

@@ -282,6 +282,8 @@ final class PhabricatorDashboardTabsPanelType
         $panel_content = pht('(Invalid Panel)');
       }
 
+      $is_selected = (string)$idx === (string)$selected;
+
       $content_id = celerity_generate_unique_node_id();
 
       $content[] = phutil_tag(