Implement Admin Endpoints

[AlperenYilmazyildiz]

IMPORTANT: Checkout to 30-admin-endpoints before implementation to avoid merge conflicts.

• [x] Implement "/api/admin/approve-seller" endpoint and implement logic as new sellers will be flagged as unapproved and will not be able to log in until it is approved.
• [x] Implement "/api/admin/delete-buyer"
• [x] Implement "/api/admin/delete-seller"
• [x] Implement "/api/admin/delete-review"