Dashboard Tabs: render "Move" buttons grayed out more consistently

Summary:
If you happen to create a Dashboard by retrieving other people's panels,
you were probably affected by this visual bug.

It seems that you cannot Move things, but you really can (and that is OK).

The reason is, to move things in a Parent Panel, you only need edit permission there,
and you do not need to also have edit permissions on every Child Panel.

Closes T15492

Test Plan:
- From user A:
    - Create two panels - editable by: User A
- From user B:
    - Create a "Tabs Panel" in a Dashboard
    - Attach the panels from User A (so, editable by, not you)

The meaningful "Move" actions should be not disabled anymore.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15492

Differential Revision: https://we.phorge.it/D25308

src/applications/dashboard/paneltype/PhabricatorDashboardTabsPanelType.php

@@ -181,7 +181,7 @@ final class PhabricatorDashboardTabsPanelType
             ->setIcon('fa-chevron-left')
             ->setHref($prev_uri)
             ->setWorkflow(true)
-            ->setDisabled(($prev_key === null) || !$can_edit));
+            ->setDisabled($prev_key === null));
 
         $dropdown_menu->addAction(
           id(new PhabricatorActionView())
@@ -189,7 +189,7 @@ final class PhabricatorDashboardTabsPanelType
             ->setIcon('fa-chevron-right')
             ->setHref($next_uri)
             ->setWorkflow(true)
-            ->setDisabled(($next_key === null) || !$can_edit));
+            ->setDisabled($next_key === null));
 
         $dropdown_menu->addAction(
           id(new PhabricatorActionView())