Fix initialization of Context in Action class.

When Action::getActionName calls Action::factory, $context
must be passed explicitly, since WikiPage does not have a getContext()
method (only Article does).

As a failsafe in Article::getContext(), use $this->page->getContext()
only if $this->page is an Article. If $this->context is null and $this->page
is not an article, fall back to RequestContext::getMain()

Change-Id: I6d8db29dfd1aaa68f3de4f8f418841205c2379af