User select autocomplete

TMS v1.x had an autocomplete function when adding students and instructors to courses (using a multiple-valued Krajee Widgets Select2 widget), which allowed searching for fragments of both Neptun codes and user names, and displayed both of them if there was a match, even before saving, thus preventing instructors from mistakenly adding wrong people. Compared to this, the v2.x UX is terrible, so this should be reimplemented.