Enable $wgExperimentalHtmlIds by default

Enable $wgExperimentalHtmlIds by default

This will result in much prettier anchors, which should actually be
intelligible in foreign languages.  For instance, for a section named
"שלום", the URL will end in "#שלום" instead of
"#.D7.A9.D7.9C.D7.95.D7.9D".  Old links will still work, since the
legacy anchors are still being output too.

This has been enabled on TranslateWiki since January, and there have
been zero complaints.  I tested just now in IE6, Firefox 3.6, Firefox
4.0, Chrome dev channel, and Opera 10.60, and all work.  I tested Opera
10.50 and IE8 some months ago as well.  A test case is at:

http://www.mediawiki.org/wiki/User:Simetrical/Id_test

(A lot of the tests are expected to fail because they already fail
anyway.)  Earlier testing indicated that Opera 10.10 wouldn't work
correctly, but that's negligible by now compared to the readability
improvements for foreign languages.  Additional testing would be
appreciated, particularly in IE7, IE8, and IE9 in their various
compatibility modes, but I hope this should be good to go by now.