Migrate PhabricatorUserLDAPInfo to PhabricatorExternalAccount

Summary: Ref T1536. This is similar to D6172 but much simpler: we don't need to retain external interfaces here and can do a straight migration.

Test Plan: TBA

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6173