Move DATETIME_FORMAT from app to app.utils

To avoid cyclical import issues

app/template_statistics/rest.py

@@ -1,11 +1,11 @@
 from flask import Blueprint, jsonify, request
 
-from app import DATETIME_FORMAT
 from app.dao.notifications_dao import dao_get_last_date_template_was_used
 from app.dao.templates_dao import dao_get_template_by_id_and_service_id
 from app.dao.fact_notification_status_dao import fetch_notification_status_for_service_for_today_and_7_previous_days
 
 from app.errors import register_errors, InvalidRequest
+from app.utils import DATETIME_FORMAT
 
 template_statistics = Blueprint('template_statistics',
                                 __name__,