Popup message notifications. (#125)

* Popup message system v1

* Networking for the popup notifications.

Ship it.

Content.Server/Interfaces/IServerNotifyManager.cs

@@ -0,0 +1,9 @@
+using Content.Shared.Interfaces;
+
+namespace Content.Server.Interfaces
+{
+    public interface IServerNotifyManager : ISharedNotifyManager
+    {
+        void Initialize();
+    }
+}