[schema] Use TZDatetime for creation_date and modification_date No work has been done for form fields/widgets since creation_date / modification_date are by default not editable through the default UI. One may want to add such feature at some point. Time will be displayed as UTC, let the end-application decide otherwise by customizing the tzdatetime printer if desired. Closes #4848923

# 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)