Made WikiPage::isRedirect() actually use page_is_redirect

Made WikiPage::isRedirect() actually use page_is_redirect

* Previously it would always fetch the whole content,
  which is silly since followRedirect() uses the redirect
  table to avoid loading the text. The initializeArticle()
  uses isRedirect() and then possibly followRedirect(). It
  makes no sense for the former to fetch all the text anway.
* The time fetching the text showed up on xenon flamegraphs.

Change-Id: I2dc216f36d3a0ea2285e64122b4d07bd9c8ae703