[views] csvexport accept an empty rset (closes #4236928) When you tried to apply the 'csvexport' view on an empty rset, the view couldn't be selected and you got a HTTP 500 error. Also add two new test cases.

[views] Display attributes in entity creation form based on "add" permission Previously, the "update" permission was used. Hence in case the latter is more restrictive that the "add" permission, an user may not be able to set such an attribute, despite she may have "add" permission on it. As a result of the change of permissions action in `editable_attributes` method (add/update depending on whether the entity is being created or modified), the "eid" attribute would have shown up in the edition form. To avoid this, it is moved in the "hidden" section (where it should arguably belong anyways). Closes #4342844.

[datafeed] Commit after all deletions in datafeed parser This avoids misleading validation error because schema constraints could be temporarily broken depending on the deletion order. Closes #4372127.

[schema] CWComputedRType is a schema type Hide it from the default views.

[doc] proofreading CWEP002 section

[CWEP002] document computed relations and attributes Related to #3546717.

[CWEP002 migration] support sync_schema_props_perms for computed attribute

[CWEP002 migration] support drop_relation_type/drop_relation_definition/drop_attribute for computed attributes

[CWEP002 migration] support add_relation_type/add_attribute for computed attributes Related to #3546717.

[CWEP002] properly handle serialization of computed attributes Until this cset, only serialization of computed attribute's formula was handled (thanks to the relation definition's properties dict). We now: * properly serialize/deserialize attribute's formula * test first if the database has been migrated and contains related column or not Related to #3546717