Support for managed attachments

[geier]

I would like to see support for the recently proposed "managed attachements" in vdirsyncer and khal, see the draft for details.

From the caldeveloper mailing list I take it, that currently only Apple's client and server, and the CMU/Fastmail Cyrus server support it, but this is obviously a chicken and egg problem somebody will need to break.

So, we could just save all attachments in a new subfolder (e.g. attachements), but would need to transform the urls to something we can save on disk.

While we are at it, we could also think about supporting Time Zones by Reference.

I'll see if I can find the time to whip a PR together.

[untitaker]

• Blocked by https://github.com/pimutils/vdirsyncer/issues/375 because we need to be able to properly read and write ATTACH properties. I don't want more code depending on the messy abstraction I built.
• This makes synchronization incredibly complex, unless we just inline all attachments into the Item (inefficient and triggers conflict resolution too often)
• Unclear how singlefile or remotestorage should deal with it
• If we store attachments as separate file in filesystem storage, we need to revert #591 and write an alternative

Some other impressions:

• Events don't appear to be able to reference the same attachment. Unsure if that's a good or bad thing
• The action query parameter is incredibly ugly.
• This RFC looks like a special-purpose solution just for the ability to read/write a single property from a collection item.

[geier]

Adding or updating attachments is going be especially fun, because of the way adding and updating attachments works from the client. But if we are having any chance of influencing how this is going to work in the future, it is now (as they are currently asking for feedback).