Support for cross-device synchronization

[parksb]

Allows users to enter the sync server an address to sync local data with the server. This requires Collie to provide the server implementation and a docker image to make it easy to run the server.

[erlend-sh]

You could do this in a very local-first way by using Iroh. Would you accept a PR for a proof-of-concept of this?

(p.s. I've sent you an email to talk more about how our projects may align)