Unify default value for restricting user enumeration with settings

If the value was never enabled or disabled, the settings show "Restrict
username enumeration to groups" as disabled. However, in some components
it was enabled by default, which caused an inconsistency in the
behaviour with respect to the settings, for example in the contacts
menu.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>