erp5_discussion: customize how to get discussion post

1. ignore size, some user has size=XX in there rss, which make catalog search versionning table
2. add limit
3. sort by modification date

bt5/erp5_discussion/SkinTemplateItem/portal_skins/erp5_discussion/WebSection_getLatestDiscussionPostList.py

 """
   Get list of latest Posts which belong to a forum (i.e. websection + predicate)
 """
+limit = 100
+kw.pop("size", None)
 # first get list of all container Threads (details should be set on predicate)
-parent_uid_list = [x.getUid() for x in context.getDocumentValueList()]
+parent_uid_list = [x.getUid() for x in context.getDocumentValueList(limit=limit, sort_on=(('modification_date', 'DESC'),))]
 
 # get sorted list of all "contained" in them Posts
 kw['sort_on'] = (('modification_date', 'DESC'),)
@@ -13,4 +15,6 @@ if len(parent_uid_list)==0:
   # no parent discussion threads therefore no posts
   return []
 
+kw['limit'] = limit
+
 return context.portal_catalog(**kw)