Shared subfloor system (#3590)

* Shared subfloor system

Will also cull the broadphase for server a lot.

* Nullable subfloor

* Snapgrid nullable

* Actually use ComponentDependency

Co-authored-by: Metal Gear Sloth <metalgearsloth@gmail.com>

Content.Client/GameObjects/Components/Disposal/DisposalVisualizer.cs

@@ -1,4 +1,5 @@
 using System;
+using Content.Shared.GameObjects.Components;
 using Content.Shared.GameObjects.Components.Disposal;
 using JetBrains.Annotations;
 using Robust.Client.GameObjects;