Browser should clear cache for API responses

Browser should clear cache for API responses

By default we send "private, must-revalidate, max-age=0" for regular
logged-in wiki viewing.  This changes API responses to match.

Later, someone should update the Cache-Control header generation so
that it works the same for API responses as it does for OutputPage.
This is becoming more important since we're using the API instead of
OutputPage for editing with VisualEditor.

Bug: T74480
Change-Id: Ib309df8568de2c7137b6d13b9ca4004150a772dd