Set "ldap" field on group creation.

This guarantees that the only way to add/remove the LDAP flag
for groups is the "omero ldap" CLI plugin. The Webclient will
automatically set "ldap" to false for newly created groups.
Updated ones will keep their previous "ldap" value.
{user,group}.py: Display "ldap" in "list".
Initial implementation of findGroupDN().
Split context mappers.