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/Interfaces/Chat/IChatManager.cs

@@ -1,6 +1,5 @@
 using Robust.Server.Interfaces.Player;
 using Robust.Shared.Interfaces.GameObjects;
-using System;
 
 namespace Content.Server.Interfaces.Chat
 {