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.Server/GameObjects/EntitySystems/StorageSystem.cs

@@ -4,6 +4,7 @@ using Content.Server.GameObjects.EntitySystems.Click;
 using JetBrains.Annotations;
 using Robust.Server.GameObjects;
 using Robust.Server.Player;
+using Robust.Shared.Containers;
 using Robust.Shared.GameObjects;
 
 namespace Content.Server.GameObjects.EntitySystems