Backport: Re-add the config options to remove the ability for users to change their displayname

Backport of https://github.com/owncloud/core/pull/6513 to stable6

Also includes https://github.com/owncloud/core/pull/6436 (the added params are used in #6513) and https://github.com/owncloud/core/pull/6318 (needed for a clean backport)

cc @karlitschek @DeepDiver1975