Respect user enumeration settings in user status lists

So far, the functions to find user statuses listed didn't respect user
enumeration settings (`shareapi_allow_share_dialog_user_enumeration`
and `shareapi_restrict_user_enumeration_to_group` core app settings).

Fix this privacy issue by returning an empty list in case
`shareapi_allow_share_dialog_user_enumeration` is unset or
`shareapi_restrict_user_enumeration_to_group` is set.

In the long run, we might want to return users from common groups if
`shareapi_restrict_user_enumeration_to_group` is set. It's complicated
to implement this in a way that scales, though. See the discussion at
https://github.com/nextcloud/server/pull/27879#pullrequestreview-753655308


for details.

Also, don't register the user_status dashboard widget at all if
`shareapi_allow_share_dialog_user_enumeration` is unset or
`shareapi_restrict_user_enumeration_to_group` is set.

Fixes: #27122

Signed-off-by: Jonas Meurer <jonas@freesources.org>