Add Mail-Reply-To support to the ReplyAuthor and ReplySmart strategies

Summary:
as recommended at https://cr.yp.to/proto/replyto.html, give the
highest priority to the Mail-Reply-To header when determining the address
of the author of a message.

Similar changes to ReplyAll are complicated enough to deserve a separate
patch.

Reviewers: knauss

Reviewed By: knauss

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D24481