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/DisposalMailingUnitComponent.cs

@@ -589,7 +589,7 @@ namespace Content.Server.GameObjects.Components.Disposal
         {
             base.Initialize();
 
-            _container = ContainerManagerComponent.Ensure<Container>(Name, Owner);
+            _container = ContainerHelpers.EnsureContainer<Container>(Owner, Name);
 
             if (UserInterface != null)
             {