Replace Arr::get($_POST|$_GET with $this->request->post|query

Kohana 3.3 now encapsulates request parameters in the request
object, so need to move away from using $_GET and $_POST
directly. These usages are the easiest, because ->post and
->query are direct replacements for Arr::get on the superglobal.
Encapsulate subrequest data into the request object, not superglobal
Favour direct array access over extract

Extract may produce unexpected "undefined variable" errors rather than
the more obvious missing array key, and causes undefined variable
warnings in the IDE.
Fix remaining references to $_POST to refer to $this->request->post