Made it possible to login using LDAP

Summary: Made it possible to link and unlink LDAP accounts with Phabricator accounts.

Test Plan:
I've tested this code locally and in production where I work.
I've tried creating an account from scratch by logging in with LDAP and linking and unlinking an LDAP account with an existing account. I've tried to associate the same LDAP account with different Phabricator accounts and it failed as expected.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, auduny, svemir

Maniphest Tasks: T742

Differential Revision: https://secure.phabricator.com/D2722