Support Bitbucket as an auth provider in Phabricator

Summary: Fixes T4251. Depends on D9761. See D9760 and discussion in D9202.

Test Plan: Authenticated using Bitbucket.

Reviewers: btrahan, asherkin

Reviewed By: asherkin

Subscribers: chad, epriestley

Maniphest Tasks: T4251

Differential Revision: https://secure.phabricator.com/D9762