R1:a12a6d5c7d1e

Add Disqus OAuth to new flows

Summary: Ref T1536. Adds Disqus as a Provider.

Test Plan: Registered and logged in with Disqus.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6165