LDAP support

model
   an account can optionally have an "external authorizer", described by
      authorizer type
         LDAP, OpenAuth
      authorizer URL
      authorizer account ID

database

Projects can support LDAP;
   this is exported in get_project_config.php

client attach
   current:
      do either lookup_account or create_account w/ email, passwd
      create account as needed
   new:
      GUI, attach form:
         "login with LDAP" checkbox
         LDAP name, password fields