Add flake.nix for Nix users

[alurm]

I removed server dependency on server/conf/server.pem, since that made Nix build.

To generate keys, make server/conf/server.pem still works.

[alurm]

I also added build: [[make "server"] ...] to cuekeeper.opam, I think it was required too.

To be clear, I don't imply that this should be merged (at least in the current form), but I thought that I should mention these changes here since they seem to be working and could be useful for someone.

[alurm]

Also, unrelated, I thought that it'd be good to have a button on the client (the one which doesn't require a server) to sync to an upstream but I don't really know how to implement it.

It would allow to both sync and have notes be local-first.

Maybe I'll send this as a separate pull request, but for now I don't even know where to start, so maybe I'll give up.

I think I found some piece of code for that in ck_model, maybe I'll try changing it.

CORS could be a problem, I suppose. I'll have to think about it.