OAuth -- add support for Disqus

Summary:
also fix some bugs where we weren't properly capturing the expiry value or scope of access tokens.

This code isn't the cleanest as some providers don't confirm what scope you've been granted. In that case, assume the access token is of the minimum scope Phabricator requires. This seems more useful to me as only Phabricator at the moment really easily / consistently lets the user increase / decrease the granted scope so its basically always the correct assumption at the time we make it.

Test Plan: linked and unlinked Phabricator, Github, Disqus and Facebook accounts from Phabricator instaneces

Reviewers: epriestley

Reviewed By: epriestley

CC: zeeg, aran, Koolvin

Maniphest Tasks: T1110

Differential Revision: https://secure.phabricator.com/D2431