Add CSS to support new HTML markup for headings with section edit
links, which will soon be used by Parsoid page views (T269630)
and by the old parser (T13555). Keep the old rules to provide
temporary support for cached page HTML and emergency opt-outs,
as well as permanent support for plain headings on special pages
and in Parsoid edit mode HTML.
See documentation page for further explanation:
https://www.mediawiki.org/wiki/Heading_HTML_changes
Depends-On: I44587461582d648b56ef0c9c7ae0c322895c69c2
Bug: T13555
Bug: T269630
Change-Id: I62d7e8ad7d8f25d145ec1f33230d6043982aec1e
