[BUG] Improve user authorization for creating assignment

[athiruma]

• QUADS version (rpm -qa | grep quads and/or quads --version):
• Python version:
• Operating System:

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https://github.com/redhat-performance/quads/blob/8055b8d6446a0b0bfbb67c5537b352d4c42a9024/src/quads/server/blueprints/schedules.py#L87-L92

Both users and admins have the same authorization levels. This will cause users can create schedules on their own

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