Multi-host integration

**is your feature request related to a problem? Please describe.**
Not related to a problem!

**Describe the idea / solution you'd like**
I think it would be really cool to have a configuration where you can point to other copyparty hosts servers from your main host and the server browser would display all files from the other servers as if it was all one server.

More speciifcally, im thinking server-a and server-b point to eachother in the configs, users can then browse content from server-a and server-b from the same UI without necessarily needing to know what files are from server-a and which are from server-b. When uploading a file you can upload the file to either server and it would show on the other server, when downloading a file it would download from the server that has the file.

This is not similar to raid or other sync/parity softwares such as ceph or gluster, the idea is to just have a fleet of servers and when you upload a file it just goes to wherever, when downloading a file it fetches from wherever the file is present.

**Describe any alternatives you've considered**
Just using a separate clustering system like ceph or gluster/seaweedfs underneath, but those are more for partity and sync than just "having a fleet of servers browseable".

**Additional context**
This would make it super easy add more hosts with more storage as time goes on, etc.. I really think its important to remember that im not asking for parity features here, since I know those are really annoying to implement/manage. We can use parity from raid or other filesystems like ceph, the idea here is really to just have a single interface that shows stuff from more than one server.

I think the easiest implementation would be to just show like server-a -> list of files, server-b -> list of files on the left side, but even cooler would be to have `file-a` -> downloads from `server-a`, `file-b` -> downloads from `server-b` and they are both together in the UI! Maybe make this an option under the share?