Make top bar link relative

WebsiteGenerator/Content/Element.swift

@@ -325,6 +325,17 @@ extension Element {
                         + pageParts.dropFirst(index)
         return allParts.joined(separator: "/")
     }
+
+    /**
+     The relative path to the site root.
+     */
+    var pathToRoot: String? {
+        guard path != "" else {
+            return nil
+        }
+        let downPathCount = path.components(separatedBy: "/").count
+        return [String](repeating: "..", count: downPathCount).joined(separator: "/")
+    }
     
     /**
      Create an absolute path (relative to the root directory) for a file contained in the elements folder.