added user_notifier gem and its configuration
adding user notification using the UserNotifier gem and moving some templates to be used by the gem
removed unused User#notify method
moved user_deactivate notification template
moved credits warning templates and fixed user_deactivate subject template
fixed observer spec
fixed user notifications
created contribution_notifications table
migrated notify calls to user_notifier gem, still need to migrate templates
moved payment_slip templates
moved refund_completed and refund_completed_slip templates
moved refund_request templates
moved refunded_and_canceled templates
moved request_refund and request_refund_slip templates
moved contribution* templates
moved confirm_contribution templates
changed PaymentNotification to use Contribution#notify_to_contributor
moved processing_payment and slip_payment_canceled templates
moved pending payment template
fixed worker specs using database checks instead of expectations