server(filesystem): rebuild everything imaginable

This wonderfully large commit replaces basically everything under the
`server/filesystem` package, re-implementing essentially everything.

This is related to
https://github.com/pterodactyl/wings/security/advisories/GHSA-494h-9924-xww9

If any vulnerabilities related to symlinks persist after this commit, I
will be very upset.

Signed-off-by: Matthew Penner <me@matthewp.io>

server/manager.go

@@ -196,7 +196,10 @@ func (m *Manager) InitServer(data remote.ServerConfigurationResponse) (*Server,
 		return nil, errors.WithStackIf(err)
 	}
 
-	s.fs = filesystem.New(filepath.Join(config.Get().System.Data, s.ID()), s.DiskSpace(), s.Config().Egg.FileDenylist)
+	s.fs, err = filesystem.New(filepath.Join(config.Get().System.Data, s.ID()), s.DiskSpace(), s.Config().Egg.FileDenylist)
+	if err != nil {
+		return nil, errors.WithStackIf(err)
+	}
 
 	// Right now we only support a Docker based environment, so I'm going to hard code
 	// this logic in. When we're ready to support other environment we'll need to make