Use the new event store when gathering user notifications when enabled

Thanks to XWIKI-17129 and XWIKI-17044 we can now start refactoring the user notifications to have fast display. The main target of this issue is to cover the user notification on the top right corner but in general use the new store API as much as possible where we can (some places will be harder to refactor and might not be included just yet because they use complex SQL request which obviously don't make sense with a no-SQL oriented API).