[erp5_officejs] Replace devellopement for global gadget by app and change signature storage for erp5 sync

Following up !308 , these are a batch of changes to fix coding style in `erp5_accounting` as reported by pylint and `Products.ERP5Type.CodingStyle`.

Also, calls to `portal_catalog.getObject` are replaced by simple `portal_catalog` calls.


/reviewed-on nexedi/erp5!309

In `sheet_per_report_section` mode if the report do not have any fields,
we use to display an empty sheet. It was confusing and not useful.