allow some logged in users to create new user accounts

The context of this change:

https://wikia-inc.atlassian.net/browse/CE-1173

Sometimes logged in users require the ability to create new user
accounts. Thereis a user right 'createaccount' that has to be respected
here. Wikia users from the 'staff' group and Wikia One users from the
'createaccount' group have that right.

@lizlux