[16.0][IMP] base_mail_sender_bcc: avoid intempestive BCC

[arnaudlayec]

Intempestive BCC like:

• When a user send its timesheet.sheet to manager's approval, the notification triggers an email to all the managers who choseed "Receive notification in email" (and not in Odoo). The user who just validated its timesheet was added in BCC of the "notification" mail
• When an external partner (e.g. customer or supplier) responds to an email sent by Odoo (e.g. a purchase request), the email is processed by Odoo and sent back to the followers of the Purchase Request. In this "re-routed" email, the "from" address, meaning the supplier, is added in BCC of the re-routed mails.

=> This PR proposes to only add Odoo users in BCC for the email sent by the mail.compose.wizard.

[arnaudlayec]

@alexis-via: a small but useful improvement, what do you think?
For the example, ALUVAL users currently refused to use this module due to the issues described, though they really want the BCC feature

[alexis-via]

With this change, I'm afraid overdue reminders will not be Bcc-ed because they don't go through the mail compose wizard.