Add a ``schema`` command to cmd ctrl to generate schema image. This changeset add the new commande and do some refactoring in cwconfig and schema.py to allow the use of CubicWebNoAppConfiguration with CubicWebSchemaLoader.

fix rset.related_entity with variables coming from subquery while some others not

3.5.12

cleanup

fix spurious http cache bug: sometimes last-modified headers is generated using non-english local, which ends up in a date that twisted can't parse and make it feels the page may be cached while it may not

simpler last_modified implementation