Integrate env abstraction layer with page objects

Implemented additional methods Environment#wiki_url and #visit_wiki for,
respectively, qualifying a relative wiki path and navigating to a
configured wiki.

Modified MediawikiSelenium::PageFactory#on_page to automatically qualify
relative-path page-object URLs (e.g. "Special:RandomPage") with the
configured mediawiki_url.

Change-Id: I857ebce57551c8f5ad1215870170b290717b9a15