[ADD] voluntary shares

I'm not proud of this MR, we should dedicate more time implementing some alternative to this.

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Changelist

• [ADD] New controller for voluntary shares using cooperator one as base /voluntary_share?odoo_company_id=4
• [ADD] Mail.template for notification of voluntary_shares
• [ADD] voluntary_share field to res.company
• [ADD] is_voluntary compute field to subscription.request
• [ADD] check field to indicate payment by SEPA