respect 'createaccount' right while requesting signup form

https://wikia-inc.atlassian.net/browse/CE-1173

The only logged in user who must not be re-routed away from the signup form are those who have the right to create new accounts (are members of the createaccount group and therefore have the createaccount right. An example use case: Wikia One's user accounts can be created by a limited group of Wikia employees who have been given the right explicitly.