Support searching for users to find their LDAP entry

Summary:

• the current LDAP auth flow expects a DN to look like cn=ossareh,ou=Users,dc=example,dc=com
• however many LDAP setups have their dn look something like cn=Mike Ossareh,ou=Users,dc=example,dc=com

Test Plan:
Test if logins work with a LDAP setup which has cn=Full Name
instead of cn=username.

To test you should ensure you set the properties needed to
trigger the search before login as detailed in conf/default.conf.php

Reviewers: epriestley

CC: mbeck, aran, Korvin

Differential Revision: https://secure.phabricator.com/D3072