[IMP] hr_expense: posting flow in multi company environment

This commit tries to fix multi company problem in Expense application. Indeed, no check
were done to ensure this business flow. Some technical decisions were made:
- journal company: the journal define the company in which the accounting entries will be
created. We know ensure with 'domains' and constraints that the journal have the same
company as the expense sheet.
- journal conditionnally required: the journal is only required when posting the expense. It
is set by defaut on creation, but can be modify by accountant users. This is why the expense
company determine the journal and not the opposite.
- company fields required: As we need to use the company from the expense / expense sheet to
determine the journal field and since expense models are business models, we needed to put
company fields required.

Business Flow
When an employee creates an expense, the expense should be in its current company. A
product from any allowed company can be chosen. Only the taxes of the expense company are
applied to compute amounts. Then a report (sheet) can be created. We ensure that the expense
lines of a report belongs to the same company as the report and to the same employee. Once
submitted and approved by manager, the account journal can be set: it is forced to be in
the same company as the expense report. The accounting entries are created in the journal
(and expense report) company. As the analytic account can be shared (company is not set)
the analytic entries will be in the expense company (handle by the accounting module).
The payment is now registered in the expense company.

Migration
As company fields are now required, fill the company fields with the company of the related
employee, or fallback on the one from the journal.

Task-1999686

closes odoo/odoo#36447

Signed-off-by: Jérome Maes (jem) <jem@openerp.com>