Fix (de)serialization of ComputedRelation read permissions For normal relation types, permissions don't need to be stored since they're just default values for the relation definitions. However, computed relations are serialized (as CWComputedRType), while their relation definitions are added at schema finalization time, and are only in memory. So add the 'read_permission' relation to CWComputedRType, and the appropriate hooks to save and restore those permissions. To avoid having to touch yams, we drop the 'add' and 'delete' permissions from the default computed relation permissions; this should probably be backported there. The actual permissions (set on the relation definitions) are hardcoded in finalize_computed_relations anyway. In deserialize_schema, the CWComputedRType handling needs to be delayed a little bit, until after we've called deserialize_ertype_permissions. The rql2sql test is adjusted because CWComputedRType has a 'name' attribute and the 'read_permission' relation, which generates ambiguity vs CWEType. We add an explicit CubicWebRelationSchema.check_permission_definitions, since we need to check both that computed and non-computed rtypes are defined properly. Based on report and initial patch from Christophe de Vienne (thanks!). Closes #5706307

# re-read ComputedRelation permissions from schema.py now that we're
# able to serialize them
for computedrtype in schema.iter_computed_relations():
    sync_schema_props_perms(computedrtype.type)