Content: Deprecate and stop using getHighlightHtml()

Content: Deprecate and stop using getHighlightHtml()

That function was clearly a bad idea, everyone should just use
getHtml() instead. It was supposed to allow extensions to extend
the content formatting process, but actually doesn't.

A better hook-based solution follows in separate commit I979e2438.

Split off from original I979e2438 and rebased with minor changes
(updated documentation comments, etc.).

Co-Authored-By: daniel <daniel.kinzler@wikimedia.de>
Co-Authored-By: Bartosz Dziewoński <matma.rex@gmail.com>
Change-Id: I76412f9d28bb145fb5975f59e538f6560e50472f