MDL-61028 core_search: Allow filtering search by user (front-end)

Creates a new 'Users' field in the search filters form. This field
requires new JavaScript and, to implement this, a new AJAX-callable
web service to search for users by name, with detailed restrictions
based on the current user's access to view profiles.

lang/en/search.php

@@ -26,6 +26,7 @@ $string['advancedsearch'] = 'Advanced search';
 $string['all'] = 'All';
 $string['allareas'] = 'All areas';
 $string['allcourses'] = 'All courses';
+$string['allusers'] = 'All users';
 $string['author'] = 'Author';
 $string['authorname'] = 'Author name';
 $string['back'] = 'Back';