Add online user counting back (ish)

PluralKit.Bot/Services/PeriodicStatCollector.cs

@@ -72,8 +72,7 @@ namespace PluralKit.Bot
             foreach (var user in guild.Members.Values)
             {
                 usersKnown.Add(user.Id);
-                
-                // Presence updates are disabled, for now we just assume every user is online, I guess
+                if (user.Presence?.Status == UserStatus.Online)
                     usersOnline.Add(user.Id);
             }