`action_post` does not always get used by odoo methods; need to plug into `_post`.

In the process, fix README & add SQL index.

Tests are independent now so their order does not matter.

Install works fine but jobs have error detection issues.

There is no more need to set an accounting date as the usual date is now used.

add new files
fix pipeline requirements

 : Reviewed migration, play_onchanges usage, and unit tests

in case other modules also install companies.

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

Follow-up on rev 7bde6eae1dcf.