Reuse the WikiPage object to save a db query

Early in the request we call Action::getActionName to get the action
name for HTMLFileCache to see if the current action is cached.
This causes data to be loaded from the WikiPage object
stored in the main request context. However, later when
we're initializing the Article object, we overwrite that
WikiPage object with the one used by the Article object.
Later on the WikiPage object has to be re-loaded, and
hence the exact same db request is run twice in one request,
which seems wasteful.

Instead, initialize the Article object using the WikiPage
object from the context, in order to save the already
loaded data.

Change-Id: I292f0d70feb505fae5fa955fd735d85ad3b22fea