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/Components/Disposal/IDisposalTubeComponent.cs

@@ -1,5 +1,6 @@
 #nullable enable
 using Robust.Server.GameObjects;
+using Robust.Shared.Containers;
 using Robust.Shared.GameObjects;
 using Robust.Shared.Maths;