Initial proof of concept implementation of MediaWiki OAuth provider

This is now configurable to work with a user-specified mediawiki url.
Borrowed some code from the JIRA provider to implement stored provider configuration.

This depends on the libphutil change I7c996145b8ce2f0453b52f9fd1fbc08e5f2b7408.


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, csteipp

Subscribers: bd808, csteipp, greggrossmeier, epriestley, Korvin

Maniphest Tasks: T5096

Differential Revision: https://secure.phabricator.com/D9203

Change-Id: Ia87baa1440c3b3d3a772660c6849547bf68a5c2a