Store user config/data on server instead of client

[DhruvaSambrani]

Data like recent notebooks etc should be stored on the server, not in the browser.
MOST of the time, this isn't an issue, but if you switch from a local instance to an ssh instance, you'd expect the local nbs to not show.

I suppose you should use Scratch.jl as in #676, but like noted, thats a Julia v1.0 thing. Until then, maybe use ~/.julia/pluto/recents.json or something like that?

[fonsp]

Can you post this suggestion to the feedback box instead?