Replace raw HTML copyright footer message with wikitext one
Replaces 'copyright' with 'copyright-footer' and 'history_copyright'
with 'copyright-footer-history' (the original still takes precedence
if set). Adds SkinCopyrightFooterMessage hook which works the same
way as SkinCopyrightFooter for the new messages. Allows disabling
the old messages by setting $wgAllowRawHtmlCopyrightMessages = false.
Co-Authored-By: Gergő Tisza <tgr.huwiki@gmail.com>
Bug: T45646
Change-Id: I5fd5607f8d43b6e934c8d4d35097cec430c56043