skin: Use Title::isKnown() / Title::exists() instead of Title::getArticleID()

skin: Use Title::isKnown() / Title::exists() instead of Title::getArticleID()

Some of these calls pre-date the existence of isKnown() and exists(), which
should be used instead of calling getArticleID() directly. For places where
we're just trying to determine link color, I used isKnown(), and for other
places we're trying to figure out where an article exists, I used exists().

Bug: T85550
Change-Id: I47c8af8485857cc0f3190b4ada1578ab672580af