[WIP] Fixed JS sort comparator

:construction: 

- added OC.Util.naturalSortComparator that uses the same algo that was
  used for the user list
- changed user list and files list to use OC.Util.naturalSortComparator
- removed toLowerCase() and changed the comparator to use
  String.localeCompare()
- added unit tests

WIP: this is only the JS part, needs to add the PHP part as well

Partial fix for #7059, #3804 

FYI @jancborchardt