Admin UI - Convert from "AngularJS" to "Angular"

With SOLR-12196 it was noted the current Solr Admin UI runs AngularJS (1.x), which is to be End-of-Life later this year. Various options were proposed for what to do next. One option is to keep the existing functionality but migrate to a newer UI framework. This ticket is to migrate the existing UI to Angular (2+).

See this readme file.