Extremely basic game ticker. (#126)

You can theoretically restart the round (resetting the map) now, but it breaks very badly if a client is connected due to various edge cases/race conditions.

Content.Server/GameTicking/GamePreset.cs

@@ -0,0 +1,10 @@
+namespace Content.Server.GameTicking
+{
+    /// <summary>
+    ///     A round-start setup preset, such as which antagonists to spawn.
+    /// </summary>
+    public abstract class GamePreset
+    {
+        public abstract void Start();
+    }
+}