erp5_administration: Implement RSS notification for Activity Failures

   This aims to allow the admin know if there is or not an activity failure on ERP5.

   It outputs at most one entry if there is at least one activity failure, based on the standard sort, (which consider priority and date) and using uid as guid, if provides a stable output even if other activities fail later on (so we dont have a flickeing output).

   DateTime.earliestTime is used to refresh dates daily (leading to new entry on most of rss readers) and to not expose when the activity was created,

   This requires no authentication.