EditPage: Add getContext() function

EditPage: Add getContext() function

EditPage already has access to a IContextSource object via
$this->mArticle->getContext(), but no code ever uses it. Add a
$this->getContext() helper function and $this->context member variable
so developers are aware that RequestContext is available and should be
used instead of globals. This is the first step to de-globalifying
EditPage.

Change-Id: I17130bdaf214e9bbe0577a0ee5564ca4760c99e1