(bug 56570) Set up 'accountcreator' user group on cawiki

Setting up a new 'accountcreator' user group on the Catalan
Wikipedia per community consensus; users will be added to
and removed from this group by administrators. See bug for
URL with on-wiki consensus for the addition of this group.

Bug: 56570
Change-Id: Icb9ea3a160772864188c64e5eae9d4dcb11bc9e0