Sync via WebDAV

[Krinkle]

I'd love to be able to have continuous backups of my subscriptions by having the app sync to a server using a well-supported standard protocol such as WebDAV.

Providers like Fastmail support this to offer an effectively Dropbox/iCloud-like functionality for backing up or syncing files across apps and devices. It is also commonly supported in self-hosted contexts either using simple Apache/Nginx configurations, or as part of larger applications like NextCloud.

For an example of an app using WebDAV for syncing small atomic transactions in this way, one could look at KeeWeb.

This way, if I lose my phone, I can simply wire this up to any other copy of NetNewsWire; or if something happened to NNW, into any other feed reader that supports OPML.

I don't currently use NNW on multiple devices, but if it could also use WebDAV for the full range of of NNW-specific sync capabilities such as read state, that'd be great for many people I'm sure. (Perhaps better as a separate issue?)

[meichthys]

This would probably resolve #2247 as well.