notifications 5.x-1.0-rc1

First RC. Multiple bug fixes and some features:
- Improved security during user switching, by moshe, #284444
- Some other bug fixes back ported from 6.x version
- Moved notifications_group to og_notifications (OG package), by Zen, #254036
- Added confirmation message when submitting subscriptions block form, #260434
- Fixed: Notifications queued when send interval is 'Never', by Scott Reinolds, #268662
- Added: PHP4 compatibility function, by phonydream2, #261902
- Added event/hook support for not saving/queueing some event types, #255402
- Removed "Duplicated subscription found and updated message", #260436
- Fixed Wrong redirection and user message not showing up after subscribing, #251979
- Reworked subscribe/unsubscribe pages, added confirmation for unsubscribe.
- Autosubscribe form_alter points to incorrent form, by Liam McDermott, #256872
- Theme username for notifications by author list, by jaydub, #253989
- Regrouped admin pages for messaging and notifications
- Fixed bug in notifications_delete_subscriptions(), by Zen, #255416
- Applied big notifications_group patch, by Zen, #254036