[16.0][l10n_br_base] commits and PRs missing in the 16.0 branch

[rvalyi]

Due to the bad migration process done by @mileo in #2164 that lost 500 early commits from the l10n_br_base module, tools like oca-ports could not be used properly with l10n_br_base to track missing commits or PRs.

I'm now redoing the l10n_br_base migration to v16 properly and this the process, while applying the current 16.0 exclusive commits using cherry-picks I got a few conflicts due to the fact that some commits or PRs were missing in the 16.0 branch for the l10n_br_base module. So far these missing commits or PRs are:

• [14.0][RFC] l10n_br_base: refactor vat field value #2718 (all commits) - [14.0][RFC] l10n_br_base: refactor vat field value
• 310da41 - [REF] l10n_br_base: Unified IE, CNPJ and CPF validations.
• 6b5d50a - [REF] faster tests with SavePointCase (TransactionCase -> SavepointCase is bad but setUp to setUpClass was good, so I'll just complete with a new commit moving TransactionCase to SavepointCase again)