Add invulnerable damage flag, godmode and damage flag commands (#1949)

* Add invulnerable damage flag and commands

* Add serialization for damage flags

* Add godmode command

Content.Shared/GameObjects/Components/Damage/DamageFlag.cs

@@ -0,0 +1,13 @@
+using System;
+using Robust.Shared.Serialization;
+
+namespace Content.Shared.GameObjects.Components.Damage
+{
+    [Flags]
+    [Serializable, NetSerializable]
+    public enum DamageFlag
+    {
+        None = 0,
+        Invulnerable = 1 << 0
+    }
+}