Introduce two new classes, containing code split off from EditPage:
* IntroMessageBuilder (edit notices and other intro messages)
* PreloadedContentBuilder (initial text of new pages / sections)
I'm doing both of these features in one change, because they share a
lot of code. They are meant to be used by alternative editors to
support all of the features of the MediaWiki edit form. This isn't
everything you need yet (we should at least do this for the edit
checkboxes too), but it's a step.
Bug: T201613
Change-Id: If0b05710cb52a977bf4e85947d72d68683a0a29e
