Project file maintenance, C# 7.2 (#108)

1. Updates all the NuGet packages (except CommandLineParser because they ruined their API)
2. Makes all projects use C# 7.2 explicitly. (not latest)
3. Use some C# 7.2 features like readonly structs and default literals.

Content.Server/GameObjects/EntitySystems/HandsSystem.cs

@@ -72,7 +72,7 @@ namespace Content.Server.GameObjects.EntitySystems
         private static bool TryGetAttachedComponent<T>(IPlayerSession session, out T component)
             where T : Component
         {
-            component = default(T);
+            component = default;
 
             var ent = session.AttachedEntity;