Epic: CalDAV Support

[RayBB]

Problem Statement

I, and others, want CalDAV support beyond what is currently supported. The purpose of this issue is to plan/track the features.

Admittedly, this is biased towards my use cases but everyone is welcome to chime in with how/why they want certain features, and even implement them.

The progress we've had so far it:

• document CalDAV status #3871
• sync title (aka summary) from SP -> CalDAV server #3895

Generally, I think we should go the direction of this integration being more like a CalDAV client that is reading and communicating with the server rather than where it is now of mostly being an import and then occasional sync out.

Here are the things I think should be next:

1. Import the description from CalDAV (It's already there but in a different field)
2. Sending the description from SP -> CalDAV (blocked on the above)
3. Once we have title, description, status, deletion all pushing SP -> CalDAV we can look into pulling updates on these things from CalDAV (this would solve some of CalDav task status and deleted tasks are not updated #2915)
4. If we delete a task in SP, delete it in CalDAV (I originally thought this would be a higher priority but Johannes pointed out it's something we'll need to be really careful as it's a big change in how things work. So for now it's much lower priority.
5. Start/Due dates, will require some research as to how they Map.
6. New Tasks (this requires substantial UI changes so it's a lower priority)
7. Tags
8. Look here to plan further steps

[github-actions]

Thank you very much for opening up this issue! I am currently a bit overwhelmed by the many requests that arrive each week, so please forgive me, if I fail to respond personally. I am still very likely to at least skim read your request and I'll probably try to fix all (real) bugs if possible and I will likely review every single PR being made (please, give me a heads up if you intent to do so) and I will try to work on popular requests (please upvote via thumbs up on the original issue) whenever possible, but trying to respond to every single issue over the last years has been kind of draining and I need to adjust my approach for this project to remain fun for me and to make any progress with actually coding new stuff. Thanks for your understanding!