LDAP: Use fully qualified DN for authentication

Use the fully qualified DN with LDAP bind for user authentication. Currently a
DN built with the base DN and uid is used. This did not work on our company
LDAP server.

Testing done: Code running successful with our company LDAP server.

Fixes bugs 2836, 3069
Reviewed at https://reviews.reviewboard.org/r/4488/
Merge branch 'release-1.7.x'