[WIP][ticket/12442] Add CSS classes to important headings

Ticket: https://tracker.phpbb.com/browse/PHPBB3-12442
Discussion: http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=45473

So far, I've only done the most important `<h2>` headings. There are many more of course, but I used these rules:
If there is no container besides `#page-body`, it should get a class (most important).
If there IS a container which is easily selectable using CSS (like `.ucp` or `.mcp`), only add a class if there are more than one of the same heading types. Apply the class to the one that is most "unique".

PHPBB3-12442