Add AFK tracking system.

Allows us to see which players have not done any input and can be considered AFK.

Content.Server/Afk/IsAfkCommand.cs

@@ -0,0 +1,36 @@
+using Content.Server.Administration;
+using Content.Shared.Administration;
+using Robust.Server.Player;
+using Robust.Shared.Console;
+using Robust.Shared.IoC;
+
+namespace Content.Server.Afk
+{
+    [AdminCommand(AdminFlags.Admin)]
+    public sealed class IsAfkCommand : IConsoleCommand
+    {
+        public string Command => "isafk";
+        public string Description => "Checks if a specified player is AFK";
+        public string Help => "Usage: isafk <playerName>";
+
+        public void Execute(IConsoleShell shell, string argStr, string[] args)
+        {
+            var playerManager = IoCManager.Resolve<IPlayerManager>();
+            var afkManager = IoCManager.Resolve<IAfkManager>();
+
+            if (args.Length == 0)
+            {
+                shell.WriteError("Need one argument");
+                return;
+            }
+
+            if (!playerManager.TryGetSessionByUsername(args[0], out var player))
+            {
+                shell.WriteError("Unable to find that player");
+                return;
+            }
+
+            shell.WriteLine(afkManager.IsAfk(player) ? "They are indeed AFK" : "They are not AFK");
+        }
+    }
+}