As recommended at https://cr.yp.to/proto/replyto.html, give the highest
priority to the Mail-Followup-To header when choosing the reply address
for "Reply to All...", and give the highest priority to the Mail-Reply-To
header when determining the address of the author of a message. (See also
D24481.)
This patch makes two other changes to ReplyAll's behaviour:
* If the KMail user was the author of the original message, then the reply
is sent from the user to the recipients of the original, not back to the
user. This is consistent with the behaviour of the SmartReply strategy.
* If the original message's To header contained addresses of people apart
from the KMail user, those addresses are added to the reply's To header,
not its Cc header. This preserves a distinction between those actively
involved in a mail thread and those who are just monitoring its progress.