[book] some more documentation and cleanups * merged existing facets documentation with the one I've just written and put almost everything within the code * added a note about __depends__ and __recommends__ and about the recommends semantic (extracted from a post on the ml) * added a note about write security checking (extracted from a post on the ml) * fixed some dumb sphinx errors

new RelationAttributeFacet + other fixes and cleanup + facets test * added documentation to Relation[Attribute]Facet and AttributeFacet explaining how to configure them * fixed behaviour of sorting for various combinaison of attribute values

[migration] remove bad test when constructing attributes to copy in rename_entity_type: the old entity type is not in the fschema due to schema deserialization trick

cleanup

[migration] fix rename_entity_type: avoid removal of attributes on the new entity type from the schema during migration

[migration] we can now specify attributes to backport to rename_entity_type using its attrs argument

[formwidgets] new allow_none attribute on BooleanField allowing a third None value for booleans

[primary view] consider rtypevid flag for relations in the 'relations' section as well

[db-fti-index] simple fix fpr progressbar-related crash when etypes is None in reindex_entities()

[views/reledit] small refactoring