notifications 6.x-2.2

Maintenance release. Bug fixes and usability improvements:
- Fixed: Wrong placeholders in subscriptions fieldset title, #513854
- Usability improvements for 'manage subscriptions' filters, #455230
- Fixed: Orphaned subscriptions not updated when deleting send intervals, #483488
- Fixed: Performance issue with user threads page, left over CAST, #517966
- Fixed: Checkbox for skip notifications showing up for disabled events, #626708
- Fixed: Notifications sent for deleted comments, by Scott Reynolds, #593218
- Added wrapper for i18n custom string translations, by aufumy, #606862
- Notifications UI: Added support for multiple forms in the same page, by bdurbin, #526924
- Fixed: Bug (typo) in notifications_load_user(), by markus_petrux, #586174
- Fixed: node autocomplete filtering issues, by DeFr, #519746
- Removed php undefined variable notices, by eMPee584, #552788
- Fixed: Wrong result with notifications_content_type_enabled('type'), #456424
- Fixed: Content-type subscription settings inconsistencies, by David Goode, #520438
- Fixed: notifications_views fixes to broken handlers, by David Goode