sort threads in topic by activity by default (bump) and add setting

apps/users.lua

@@ -261,7 +261,9 @@ app:post("user_settings", "/:username/settings", function(self)
   if me.id ~= target_user.id then
     return {redirect_to = self:url_for("user", {username = self.params.username})}
   end
-
+  if self.params.topic_sort_by == "activity" or self.params.topic_sort_by == "thread" then
+    self.session.sort_by = self.params.topic_sort_by
+  end
   local status = self.params.status:sub(1, 100)
   local original_sig = self.params.signature or ""
   local rendered_sig = babycode.to_html(original_sig, html_escape)