Paginate the function to get all complaints

The DAO function to get all complaints has been paginated and the
endpoint which uses it has been updated to take a page number as a query
parameter.

app/dao/complaint_dao.py

@@ -1,5 +1,6 @@
 from datetime import timedelta
 
+from flask import current_app
 from sqlalchemy import desc
 
 from app import db
@@ -13,6 +14,15 @@ def save_complaint(complaint):
     db.session.add(complaint)
 
 
+def fetch_paginated_complaints(page=1):
+    return Complaint.query.order_by(
+        desc(Complaint.created_at)
+    ).paginate(
+        page=page,
+        per_page=current_app.config['PAGE_SIZE']
+    )
+
+
 def fetch_complaints_by_service(service_id):
     return Complaint.query.filter_by(service_id=service_id).order_by(desc(Complaint.created_at)).all()