(Bug 52412) dismissing copyright warning no longer bound to interface language

Introduced a new special message, "wikibase-shortcopyrightwarning-version". This message is an
indicator for what version of the copyright warning is prevailing. This message is used for
configuration purposes and should therefore not be translated or duplicated in any other languages.
Should most likely be changed via the [[Mediawiki:wikibase-shortcopyrightwarning-version]] page if
the wiki's license for contents gets changed or if "wikibase-shortcopyrightwarning" is overwritten
on the [[Mediawiki:wikibase-shortcopyrightwarning]] page. Changing this message will result into
all users seeing copyright warnings again even if they have dismissed them previously.

Also replaced string mw.config var "wbCopyrightWarning" with "wbCopyright", an object with
"version" and "messageHtml" fields.

Change-Id: I0e4f4a79458d1f4bb5e349a19f0b6d645bec6b09