[IMP] account: Improve lock dates

The tax lock date no longer blocks the user when creating a journal entry priors to it.
Instead, the accounting date is moved automatically during the validation to the next
available date.

This commit also brings more flexibility when dealing with the tax lock date. Indeed,
the error is now shown only when adding something impacting the tax report that was not
always the case before (e.g. adding a new line not affecting the tax report must not
raise something).

Others check has been also refactored to be more consistent with the "fully editable" mindset.

closes odoo/odoo#36304

Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>