initial proof of concept implementation of mediawiki oauth provider

Summary: The second part of T5096, see also D9202 for the libphutil oauth adaptor

Test Plan: enable mediawiki oauth, attempt to log in via mediawiki labs account

Reviewers: #blessed_reviewers, epriestley

Subscribers: bd808, csteipp, greggrossmeier, epriestley, Korvin

Maniphest Tasks: T5096

Differential Revision: https://secure.phabricator.com/D9203
Merge branch 'arcpatch-D9203'