Implement Multi-backend server support

[Dan-megabyte]

Now for the big one!

A full revamp of the installer/bootstrap script, which in the future could lead to support for setting up more than one server...

• Moved installation-only files from contrib/ to installer/, to organize the folder hierarchy and reduce the clutter in contrib/
• Moved backend minecraft server from server/ to servers/backend/, to support multiserver setups
• Future updates to installer could prompt users for the server names, and auto number the ports they will be using.

Also:

• The root deploy/update.sh now loops through all the folders in servers/ in addition to proxy/, calling the update.sh in each of them.
• Added wget to the bootstrap requirements because we use it to fetch the server files (velocity.jar AND paper.jar)

With this PR, to set up another server with name <SERVER_NAME>, just:

• change into the deploy directory as minecraft
• cp -r installer/default_config/server servers/<SERVER_NAME>
• add another server entry to proxy/plugins/vane-velocity/config.toml and proxy/velocity.toml
• configure your server
• make sure you chose and configured a different port for your server (25502, for instance)
• run servers/<SERVER_NAME>/update.sh to get server files
• as root, systemctl enable --now minecraft-server@<SERVER_NAME>
• join and /server <SERVER_NAME>
  You can start and stop any server with the minecraft-server@<SERVER_NAME> service!

However, server administrators now need to use sudo minecraft-attach server-backend instead of just sudo minecraft-attach server. This is to differentiate between the proxy (velocity) and a backend server (paper) named proxy.

As always, welcome to any edits or suggestions!