Provide SSO backend for external systems

[naxxfish]

Membership system should be able to act as a single point of authentication for various systems one might want to use in a makerspace. Many systems support LDAP for authentication, so perhaps we should implement one of these ?

Ldapjs might work well.

[naxxfish]

Rather than specifying LDAP, we should think a bit more about how we could use membership-system as a way of providing single-sign on for any site within the domain - perhaps via SAML or similar technology.