Shared Containers (#3331)

* Namespace changes for containers.
Moved ContainerSlot from content to engine.

* Merged client/server ContainerManagerComponents into a single shared version.

* Mapfile and nullability fixes.

* Upgrades map.

* Update engine.

Content.Client/Utility/UnobstructedExtensions.cs

@@ -1,6 +1,7 @@
 using Content.Shared.GameObjects.EntitySystems;
 using Content.Shared.Physics;
 using Robust.Client.Player;
+using Robust.Shared.Containers;
 using Robust.Shared.GameObjects;
 using Robust.Shared.IoC;
 using Robust.Shared.Map;