use @context for magic urls rather than @page.owner

This way we can have
url_for @context -> /the-true-levellers
url_for @group   -> /groups/the-true-levellers

This allows us to nest urls like
url_for @group, :settings
url_for @context, @page
test contexts_controller rather than dispatch_controller

the latter is only remaining as a super class to inherit from for controllers that dispatch.