[FIX] mail.thread: correct matching when finding author + test

The previous matching rules were too fuzzy and allowed random
prefix-match or tail-match of other user's emails.
For example when looking up a partner matching 'foo@bar.com'
the system would sometimes find 'dom.foo@bar.com' instead,
or 'foo@bar.com.tw'.

Fixed by only allowing direct case-insensitive email match
of an addr-spec, or substring match of the addr-spec enclosed
in angle brackets, within a name-addr pair.
See also RFC5322, section 3.4

Also adapted related message_find_partner_from_emails() method
to factor out the partner email resolution mechanism to avoid
the same problem.

Adds corresponding regression test.