Fix for bug 4469: Implement MediaWiki:<namespace>:Sitenotice

Fix for bug 4469: Implement MediaWiki:<namespace>:Sitenotice
MediaWiki:Namespacebanner contains the "base name", to which the name of the namespace is appended, like "basename-namespace". By default the base name is also "Namespacebanner" to add confusion. In namespace names, spaces are replaced by "-" (due to some weird message handling bug).
So the namespace-specific notice messages loke like:
"Namespacebanner-" (main namespace)
"Namespacebanner-Talk" (Talk namespace)
"Namespacebanner-MediaWiki-talk" (for MediaWiki_talk)