allow some logged in users to create user accounts

The context of this change:

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.
fix a typo in the right name
add comments
Merge pull request #5643 from Wikia/dev-ce-1173-cannot-create-accounts-on-wikia-one

allow some logged in users to create new user accounts