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.IntegrationTests/ContentIntegrationTest.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using System.IO;
 using System.Threading.Tasks;
 using Content.Client;