Refactor model.py into separated model files

[gormless87]

Describe the task
refactor model.py (under /models folder) into separated model files (as [domain_xyz]_model.py) since it is getting bigger. And as result, we should consider providing summary/reasons to long-standing tickets #631 and #1028 and then closing these tickets.

Acceptance Criteria

• Separate model.py in both "server/backend" and "server/admin_management" into smaller model files.
• second
• third

Additional context

• Split this from Standardize datetime configuration on the backend #1647