Typo, redundant string interpolation, namespaces and imports cleanup (#2068)

* Readonly, typos and redundant string interpolations

* Namespaces

* Optimize imports

* Address reviews

* but actually

* Localize missing strings

* Remove redundant vars

Content.Shared/GameObjects/EntitySystems/Atmos/GasOverlayChunk.cs

@@ -1,8 +1,4 @@
-using System;
-using System.Collections;
 using System.Collections.Generic;
-using Robust.Shared.Interfaces.Timing;
-using Robust.Shared.IoC;
 using Robust.Shared.Map;
 using Robust.Shared.Timing;
 using Robust.Shared.Utility;