R1:92861cc1f000

Dashboard Tabs: fix action "Move Right"

Summary:
It seems that if you use the "Move Right" action, you can get
an unrelated error message about "Move Left":

This is already the first tab. It can not move any farther to the left.

This happened because there was an incomplete condition.

Closes T15493

Test Plan:
- visit /dashboard/panel/ and create at least two panels (please, owned by you)
- visit a Dashboard, and add a Tabs Panel
- take the first Tab: "Move Right"

After this change, "Move Right"…