[WIP] Add In-Reply-To header to email notifications

[fguillot]

No description provided.

[lukasjuhrich]

I think this would be a great improvement!

To build on this: wouldn't it be possible to set a reasonable Message-ID? Otherwise, I'm not sure how this is supported with standard mail-clients.

For instance, github uses the following scheme for the Message-ID and In-Reply-To headers:

• The „root event“ (creation) has a message-ID of the form <org/project/object_type/object_number@github.com> (think object_type=issue), so we could use something like <project-id/task-id@$kanboard_server>
• Subsequent events set the (deterministic) Root-Event Message-ID Header in In-Reply-To and have Message-IDs of the form <org/project/object_type/object_number/event_type/event_number@github.com>. I don't think the distinction of event types is necessary here, and would propose to append the timestamp to get a unique value that still clearly relates to the root Message-ID.

[fguillot]

Abandoning this PR. I never took the time to finish it.