Support BitBucket as an auth provider

Summary: Ref T4251. See D9760, which fixes the root issue. See D9202 for discussion.

Test Plan: Authenticated with BitBucket.

Reviewers: btrahan, asherkin

Reviewed By: asherkin

Subscribers: epriestley

Maniphest Tasks: T4251

Differential Revision: https://secure.phabricator.com/D9761