Ajaxify user list for files external

replaces chosen with select2, fixes #7499

@DeepDiver1975 @blizzz @PVince81 please review

If I understand well , since this is in `initSelection` we are checking whether every user specified in the input field (previously selected) really exists, and if yes we add them to the selection for select2.

And it seems that we need one ajax call for every user/group we want to validate.