(bug 41729) Move section edit links to after the headings

This requires minor changes in various parts of MediaWiki, and
being extra careful about cached rendered pages' HTML.

Fun fact: editsection links are not made in Parser. They're made in
Linker, in Skin *and* in ParserOutput.

Client-side code and screen-scrapers will have to be adjusted to
handle both cases (old HTML will still be visible on cached page
renders until they are purged); extensions using the DoEditSectionLink
or EditSectionLink hooks might need adjustments as well.

* Linker: Change the HTML of pages to move the link itself from the
  beginning of the heading (before <span class="mw-headline">) to the end
  of the  heading (after the span).
* Skin: Change the class from .editsection to .mw-editsection; we use this
  opportunity to clean up old cruft, and this makes it much easier to
  handle cached renders (by just detecting the old class).
* ParserOutput: Implement a horrible hack to support cached parser
  outputs with the old order of items.
* Ensure everything that should support both classes supports both
  classes (this includes print stylesheets and some scripts).
* Implement styles for the new look for all the skins (did this in
  shared.css; the styles are non-intrusive and can be overridden
  easily, and all of the skins were using the same look before).

Change-Id: I6a6c12a90de3604012420b20c1f520e0ece170ab