[IMP] mail: improve returned values of _message_compute_author

Purpose of this commit is to ease the use of mail.thread tool method
_message_compute_author. Instead of returning a dict with two values and
having to unpack them, directly return a tuple with both author and
email_from values. All calls and return management are the same in mail.
Let us simplify code.

Task ID 1853147
PR #32243