Remove unnecessary conditions related to invoices

in account.move model

This commit consolidates the logic for updating date fields in the
`account.move` model into a single private method `_update_dates`.

Docstrings

only if a fiscal year of same code exists for the same company. The exclusion
must be executed on the basis of the company and not without it.

On call of the 'find' methods of fiscal years and accounting periods, ensure
that only one object is returned.

to specify a keyword argument, which will define the codes of the periods to
create.

a recordset, and not only one record at once.

This would happen when running tests on a DB where ``account_period`` is
already installed.

Follow-up on rev e9cc0fe9ab5f.

obsoleted by accounting date this module adds.

Upgraded the module version.
Updated the change log.

based on menu items.

Also update tests in this module to use them instead of creating them anew.

To be reworked.

or an accounting document.
Updated the translations.

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.