Fix, refactor model classes, add Views and edit database

Add views for admin and employee
Add popup views for admin and employee
Add order status and order type
Add error popup
Fix typo in OrderItem class
Add StringToSlug class to convert string to slug
Refactor Customer, Employee, FoodCategory, FoodItem, Order, Session classes
Update create_new_table.sql and insert_data.sql (delete rebate, finalAmount in order table)

PTIT-D22KH / milktea_sales_management_project