tickets#show: use `created_at` rather than `updated_at` as date of the replies.
tickets#show: use `created_at` rather than `updated_at` as date of the replies.

Otherwise, after importing and merging tickets, this shows „1 minute
ago“ even if the emails are older than that.

https://trello.com/c/ePR0mZFT/1145-brimir-e-mails-importieren
(cherry picked from commit 015e345f13523c3680a2996854d342fd65a0aeab)

ivaldi/brimir,fiedl/brimir,fiedl/brimir,ivaldi/brimir,ivaldi/brimir,ivaldi/brimir,fiedl/brimir,fiedl/brimir