Specifics to play better with accounting changes:

* No longer change the base ``date`` field; add ``accounting_date`` instead.
* Build periods when loading accounting data for a new company.

Remove a "special" field & rename an action.

Updated the translations.
Executed "black" and "isort".

Simplify the code in the process to iterate on each accdoc one at a time (as we
now have a behavior (cut-off) that depends on journal types).