[schema] Add composite on a few relations

The following behaviors are expected :

*   ExternalAuthProvider deletion should delete the linked ExternalAuthService
    and ExternalIdentity.

*   OAuth2Session should be deleted if its 'external_identity' or 'service' is
    deleted

migration/0.2.0_Any.py

+sync_schema_props_perms('provider')
+sync_schema_props_perms('identity_of')
+sync_schema_props_perms('service')

schema.py

@@ -69,7 +69,8 @@ class ExternalAuthService(EntityType):
             'delete': ('managers',)
         },
         cardinality='1*',
-        inlined=True
+        inlined=True,
+        composite='object'
     )
     application_name = String(
         maxsize=64, description="Application name",
@@ -96,7 +97,8 @@ class ExternalIdentity(EntityType):
             'read': ('managers', 'users'),
             'add': ('managers',),
             'delete': ('managers',)
-        }
+        },
+        composite='object'
     )
     identity_of = SubjectRelation(
         'CWUser', cardinality='?*', inlined=True,
@@ -104,7 +106,8 @@ class ExternalIdentity(EntityType):
             'read':   ('managers', 'users',),
             'add':    ('managers',),
             'delete': ('managers',)
-        }
+        },
+        composite='object'
     )
     uid = String(
         required=True,
@@ -137,7 +140,8 @@ class OAuth2Session(EntityType):
             'read':   ('managers', 'users',),
             'add':    ('managers',),
             'delete': ('managers',)
-        }
+        },
+        composite='object'
     )
     access_token = String(required=True, unique=True)
     active = Boolean(required=True, default=True)