[IMP] Withholding taxes

[oco-odoo]

// Commit 1
[IMP] account: Refactor tax group widget with OWL

Refactor the tax group widget with the new JavaScript Odoo Framework:
OWL. This widget has 2 different components, one representing a tax group
and one representing the list of tax groups.

Task ID: #2312900
Initial Task ID: #1986412

// Commit 2
[IMP] account: allow mixing cash basis and non-cash basis taxes on the same line

The tax_exigible field of account.move.line made it impossible to mix 'on_payment' and 'on_invoice' taxes on the same invoice line, as the base line couldn't be be tax exigible and non-tax exigible at the same time. However, this use case is needed by some countries in order to implement withholding taxes.

To solve that, we totally remove the tax_exigible field from account.move.line and instead compute it on the fly with a domain (also passed to the query_get for SQL queries). An 'always_tax_exigible' stored computed field is also added on account.move, in order to still allow (like before) putting cash basis taxes on a miscellaneous operation without any payable/receivable line and still see it become exigible without needing any payment.

[IMP] account: improve cash basis traceability

A smart button is now available on invoices generating cash basis entries, so see them all at once. The move originally creating cash basis entries is also shown as a field on them.

// Commit 3
[IMP] account, sale, purchase, l10n_latam, l10n_ar: add the possibility to apply taxes after the invoice total, for withholding

It is now possible to check a boolean on the tax group, making its taxes appear below the 'Total' value on invoices, PO and SO. The tax will also be excluded from the taxed amount. This is intended for withholding taxes, which can now be implemented with negative taxes and a tax group with this field to True.

To do that, this commit entirely refactors the way amount_by_group worked, and replaces it with a more complete json field called tax_totals_json. It also streamlines the way taxe totals are displayed on invoices, PO and SO and makes it so that a common code is called instead of copy-pasting the same block 3 times as before.

[IMP] purchase: always display tax totals by groups on purchases orders

Before, tax totals on purchase.order's form were not shown by group, and were instead all aggregated in a single "Taxes" category. The same went for the pdf export. The portal view, though, did show the totals by group. We now display the tax groups in the same way all the time.

Task: 2457374

[robodoo]

[sdegueldre]

Lots of changes in /web, thankfully we have codeowners to spot those sneaky changes!

[qdp-odoo]

partial review on the tax_exigible field removal.