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/HandsSystem.cs

@@ -12,6 +12,7 @@ using Content.Shared.Interfaces;
 using JetBrains.Annotations;
 using Robust.Server.GameObjects;
 using Robust.Server.Player;
+using Robust.Shared.Containers;
 using Robust.Shared.GameObjects;
 using Robust.Shared.Input.Binding;
 using Robust.Shared.Localization;