WikiPage: Use Title::isKnown() in hasViewableContent()

WikiPage: Use Title::isKnown() in hasViewableContent()

The current implementation of hasViewableContent() is basically the same
as Title::isKnown(), except that it switched the order of
isAlwaysKnown() and exists(). (Also it used WikiPage::exists() instead
of Title::exists(), but they're functionality equivalent).

This will make refactoring Title::isAlwaysKnown() easier in the future
as there is one less caller.

Change-Id: I698f08fb0f3e6c3bc702ec7d523d7eda063e18ca