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.Server/GameObjects/EntitySystems/DoAfter/DoAfterEventArgs.cs

@@ -1,10 +1,8 @@
 #nullable enable
 using System;
 using System.Threading;
-using Content.Shared.GameObjects.EntitySystems;
 using Content.Shared.Physics;
 using Content.Shared.Utility;
-using Robust.Shared.GameObjects.Systems;
 using Robust.Shared.Interfaces.GameObjects;
 
 // ReSharper disable UnassignedReadonlyField