Add initial niaeve implementation of SFTP logging

This will end up flooding the activity logs due to the way SFTP works, we'll need to have an intermediate step in Wings that batches events every 10 seconds or so and submits them as a single "event" for activity.

server/activity.go

@@ -16,7 +16,12 @@ type ActivityMeta map[string]interface{}
 const ActivityPowerPrefix = "server:power."
 
 const (
-	ActivityConsoleCommand = Event("server:console.command")
+	ActivityConsoleCommand      = Event("server:console.command")
+	ActivityFileDeleted         = Event("server:file.delete")
+	ActivityFileRename          = Event("server:file.rename")
+	ActivityFileCreateDirectory = Event("server:file.create-directory")
+	ActivityFileWrite           = Event("server:file.write")
+	ActivityFileRead            = Event("server:file.read")
 )
 
 var ipTrimRegex = regexp.MustCompile(`(:\d*)?$`)