Actually define jMY date format for Arab

Followup to I5d4a799b0 (10b6c22c45): testers failed to notice that
no new format was actually added to Special:Preferences.
Now it is and it's also used correctly e.g. in action=history
(verified by means of ctrl-c ctrl-f ctrl-v and {{#time:j M Y}}).

Bug: 65245
Change-Id: I70de62c859b9c637f989d68bc86e73113273aff8
Merge "Actually define jMY date format for Arab"