[IMP] mail: add method to send an email on activity mixin
[IMP] mail: add method to send an email on activity mixin

Purpose of this commit is to have a shortcut allowing to send an email based
on an email template in the activity mixin. Its use will be to integrate
the sending of emails directly in chatter-activity widget.

This commit is linked to task ID 1872169 and PR #26185.

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