Changes for broadcast and community notification

Covers following issue/features -

Avoid multiple calls for member api and group api
Fix for duplicate notifications (challenge api)
Stopping notification for "A new submission is uploaded" events (submission.notification.create)
Changes as per described at topcoder-platform/community-app#4114
Public group logic changes - now excluding group like ["!CS", "!Zurich", "!Topgear"]