Introduce new global config to disable user enumeration - first to be us...

...e in the sharing drop down - more to follow ???

@karlitschek to be used in certain education setups

@schiesbn @PVince81 @LukasReschke  please review - THX
I'd add the following disclaimer block:

> As user enumeration the ownCloud project considers features such as auto-complete that allows adversaries to enumerate users without knowing their full user- or displayname. However, we do *NOT* consider it as user enumeration if the adversary provides the complete username and those (non-)existance is leaked. 
>
> Please notice that the ownCloud project only guarantees the correct implementation of this feature for the following components: "core", "files", "files_sharing", $insertApp. For all other shipped applications (for example "calendar") the implementation of this feature is happening on a best effort base of the responsible maintainer. One should make in no circumstance any assumptions that enabling this option switch will help preventing user enumeration in those applications. The ownCloud project will it consider *NOT* as security bug if user enumeration is still possible in these applications even when this switch has been set to "false".