Updated page metadata to include in tour

The Traits page wasn't being included in the tour TOC, but was being referenced by the tour pages before and after it, Classes and Class Composition with Mixins respectively. Updated the page metadata to include it in the tour, and fix the other tags.

Author: bradfordlynch

_tour/traits.md

@@ -4,12 +4,15 @@ title: Traits
 
 discourse: true
 
-tutorial: scala-tour
-categories: tour
+partof: scala-tour
+
 num: 5
 next-page: mixin-class-composition
 previous-page: classes
-assumed-knowledge: expressions, classes, generics, objects, companion-objects
+topics: traits
+prerequisite-knowledge: expressions, classes, generics, objects, companion-objects
+
+redirect_from: "/tutorials/tour/traits.html"
 ---
 
 Traits are used to share interfaces and fields between classes. They are similar to Java 8's interfaces. Classes and objects can extend traits but traits cannot be instantiated and therefore have no parameters.