Correctly sort posts during generation

CHDataManagement/Generator/Post Lists/PostListPageGenerator.swift

@@ -32,11 +32,14 @@ final class PostListPageGenerator {
         "\(source.pageUrlPrefix(for: language))/\(pageNumber).html"
     }
 
+    /**
+     Create pages for the given posts, sorted by date (newest first)
+     */
     func createPages(for posts: [Post]) {
         // Sort by newest first, filter drafts
         let posts = posts
             .filter { !$0.isDraft }
-            .sorted { $0.startDate > $1.startDate && $0.id < $1.id }
+            .sortedByStartDateAndId()
 
         let totalCount = posts.count
         guard totalCount > 0 else {

CHDataManagement/Model/DateItem.swift

@@ -13,6 +13,10 @@ protocol DateItem {
 
 extension Sequence where Element: DateItem {
 
+    /**
+     Sort the elements first by their start date (newer first),
+     and then by id, if the start date is equal.
+     */
     func sortedByStartDateAndId() -> [Element] {
         sorted { (lhs, rhs) -> Bool in
             if lhs.startDate == rhs.startDate {