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/Components/Projectiles/FlashProjectileComponent.cs

@@ -1,4 +1,3 @@
-using System;
 using Content.Server.GameObjects.Components.Weapon;
 using Robust.Shared.GameObjects;
 using Robust.Shared.GameObjects.Components;