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