Partner multi-company

This module replaces the single-company setting on contacts with a list of companies. On the contact form, in the Sales & Purchase tab, the old company field is hidden and a new "Companies" tag field appears (only for users allowed to work with several companies). You can pick several companies for one contact, or leave it empty so every company can use it.

• Contacts that belong to a parent company cannot change their own company list; they inherit it from the parent.
• When a user is created or their allowed companies change, those companies are automatically added to that user's contact, so the user always sees their own contact.
• A check blocks saving a contact if it is linked to a user and does not include all the companies that user is allowed to work in.
• When a new company is created, it is added to the contacts of its users.
• Bank accounts no longer show their own company field; they automatically take the companies of their contact, and a bank account can be used by a company that is a parent of one of those companies.

Access rules are adjusted at installation so contacts and bank accounts are visible when they are shared with the current company or have no company restriction. Existing contacts keep their original company, and users' contacts are completed with any missing companies. Uninstalling restores the standard rules.