configure account creation behavior

introduce new config parameter account_creation disabled | admin | public (replaces self registration property).

user creation is either: disabled, allowed for privileged users, allowed for all

default when not set: disable new account creation through sign-up plugin

all functions related to account creation check the value before creating users

respective links/button/texts in HTML should be hidden if account creation is disabled.

respective websites are not delivered (eg. http://localhost:9080/sign-up/token-info )

reset password functionality is unaffected by this switch (and should not be reused for any of the above functionality).