[backport 11.0] implement custom HtmlSerializers to better control character entity encoding

Extend the 3 HtmlSerializers (Compact, Simple, Pretty) and override their org.htmlcleaner.HtmlSerializer#escapeText with our own implementation. Our implementation will implement the same conversion rules as CKEditor does using the default product configuration. E.g. convert all character entities to the "normal" Unicode code point except for nbsp, gt, lt, amp.