Extract and order the RSS threadmark list by published date rather th…

…an assume

that the last in the list is the latest.  This fixes the issue of the
threadmarks being reordered to have the most recent one at the top.

NetTally.Core/Input/Forums/Adapters/XenForo1Adapter.cs

@@ -260,11 +260,16 @@ public async Task<ThreadRangeInfo> GetStartingPostNumberAsync(IQuest quest, IPag
                     var items = channel.Elements(itemName);
 
                     XName titleName = XName.Get("title", "");
+                    XName pubDate = XName.Get("pubDate", "");
 
                     var filteredItems = from item in items
                                         let title = item.Element(titleName).Value
                                         where !((quest.UseCustomThreadmarkFilters && quest.ThreadmarkFilter.Match(title)) ||
                                                 (!quest.UseCustomThreadmarkFilters && DefaultThreadmarkFilter.Match(title)))
+                                        let pub = item.Element(pubDate).Value
+                                        where string.IsNullOrEmpty(pub) == false
+                                        let pubStamp = DateTime.Parse(pub)
+                                        orderby pubStamp
                                         select item;
 
                     var lastItem = filteredItems.LastOrDefault();