Add timestamp of update to email notifications

Adding a comment, e.g., during pull request review, triggers an email reaching the owner of the pull request. These emails can cause confusion, as they show no history and can arrive with delay.

Easy improvement: add the timestamp of the change to notification emails.

In general, I would apprechiate a history mechanism regarding comments, also linking comment versions to revisions. But that is, I guess, a major effort. Just adding a timestamp should be easily doable.

BTW, a similar issue is present with Jira