on creation only test for existing users if the backend supports user creation
this solves the issue where no users can be created any more if backends are active which always return true on userExists() like WebDAV Auth
Merge branch 'master' into fixing-1424-master
no need to implement createUser in web dav auth
Merge branch 'master' into fixing-1424-master
introduce new virtual method hasUserListings() to determine if a user backend can list it's users.
https://github.com/owncloud/core/pull/1598#discussion_r2976684
Merge branch 'master' into fixing-1424-master

Conflicts:
	settings/js/users.js
missing piece after merge
Merge pull request #1598 from owncloud/fixing-1424-master

Fixing 1424 master