Configure external user via LDAP group

Proposal

Allow the configuration of a user's "external user" to be determined by their LDAP group.

We are using LDAP for authentication within our company and we have external users (contractors/interns) which we'd like to restrict access to internal projects. These external users however get allocated standard email addresses so the "Internal users" regex on the email address won't be able to differentiate internal and external users.