Change erroneous loose comparison to strict comparison in tab panels

Summary:
Fix a loose comparison causing a bug when comparing the selected tab (0) with a tab which has an alphanumeric ID which doesn't begin with an integer.

E.g., `(0 == 'kq3p37awi2n5')` is true in PHP 7.4 and below, this can sometimes cause multiple tabs to be displayed when a tab panel is first loaded onto a page:

{F278735}

Test Plan: Create a tab panel with at least 3 tabs, add a couple more and ensure that multiple tabs aren't loaded.

Reviewers: O1 Blessed Committers, valerio.bozzolan, Cigaryno, avivey

Reviewed By: O1 Blessed Committers, valerio.bozzolan, Cigaryno, avivey

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15146

Differential Revision: https://we.phorge.it/D25067

src/applications/dashboard/paneltype/PhabricatorDashboardTabsPanelType.php

@@ -111,9 +111,11 @@ final class PhabricatorDashboardTabsPanelType
         $name = pht('Unnamed Tab');
       }
 
+      $is_selected = (string)$idx === (string)$selected;
+
       $tab_view = id(new PHUIListItemView())
         ->setHref('#')
-        ->setSelected((string)$idx === (string)$selected)
+        ->setSelected($is_selected)
         ->addSigil('dashboard-tab-panel-tab')
         ->setMetadata(array('panelKey' => $idx))
         ->setName($name);
@@ -286,7 +288,7 @@ final class PhabricatorDashboardTabsPanelType
         'div',
         array(
           'id' => $content_id,
-          'style' => ($idx == $selected) ? null : 'display: none',
+          'style' => $is_selected ? null : 'display: none',
         ),
         $panel_content);