Expose all mail as a CalDAV list, with each message a task.

[RokeJulianLockhart]

Is your feature request related to a problem? Please describe.

Currently, I utilise labels to track completion of mail. However, I would prefer to be able to utilise NC's task management infrastructure in its stead, because every message I receive via e-mail is a task.

Describe the solution you'd like

Expose all mail as a CalDAV list, with each message a task. Completion states can be trivially exposed as labels to third-party mail clients.

Additional context

This does not duplicate to issues/6127. I consider what it proposes to be a workaround at best, especially because it means that:

1. as issues/11255 describes, two objects then exist

2. the user must create a task for all mail, whereas, for me, every message is, always, a task