Actual lockers (#195)

Adds storing entities into lockers the way we all know and love.
Relies on an implementation of ITileDefinition in https://github.com/space-wizards/space-station-14/pull/193 (just like origin/master)
#191

Content.Server/GameObjects/Components/PlaceableSurfaceComponent.cs

@@ -0,0 +1,20 @@
+using Robust.Shared.GameObjects;
+using Robust.Shared.Serialization;
+
+namespace Content.Server.GameObjects.Components
+{
+    public class PlaceableSurfaceComponent : Component
+    {
+        public override string Name => "PlaceableSurface";
+
+        private bool _isPlaceable;
+        public bool IsPlaceable { get => _isPlaceable; set => _isPlaceable = value; }
+
+        public override void ExposeData(ObjectSerializer serializer)
+        {
+            base.ExposeData(serializer);
+
+            serializer.DataField(ref _isPlaceable, "IsPlaceable", true);
+        }
+    }
+}