Prepping for site default overrides of copyright/TOS statements for Wikimedia

Prepping for site default overrides of copyright/TOS statements for Wikimedia
sites with updated CC-BY-SA license... Messages now available in
WikimediaMessages extension for localization:
* wikimedia-copyright - page footer
* wikimedia-copyrightwarning - edit page, above summary/save button
* wikimedia-editpage-tos-summary - edit page, below summary/save button & above edittools

Initial English defaults are taken from
http://meta.wikimedia.org/wiki/Licensing_update/Implementation

Ariel's search for other copyright-related messages looks fairly clean,
so we should be good here for now as $wgRightsText/$wgRightsUrl are
already updated.

Messages are overridden using new hooks, which can also be used to switch the
messages based on title or namespace -- this may be desirable for sites such
as mediawiki.org which keep some namespaces PD etc.

Use of the overridden messages by WikimediaMessages is switched in when
$wgRightsUrl is set to 'http://creativecommons.org/licenses/by-sa/3.0/'