move ldap-related code to its own cube
do we want to keep cubicweb/server/sources/ldapfeed.py
and cubicweb/sobjects/ldapparser.py
in the core or do we want to extract it to a cube to make the core simpler ?
now that we tend to use OAuth everywhere instead of LDAP, we could revisit the choice to put LDAP in the core. oauth is a cube.