* Simplify message existence checks by using wfMessage() instead of wfMsg() with...

* Simplify message existence checks by using wfMessage() instead of wfMsg() with wfEmptyMsg()
* Fixed one check in Skin::addToSidebarPlain() that used user language for existence and content language for message's content
* Changed SkinTemplate::buildContentNavigationUrls() to use Title::getDefaultMessageText() instead of wfEmptyMsg()