[Backend] Convert API endpoints to rest framework

*** This task must be completed before the unit testing for backend task (#114 )

Currently, most of our API endpoints are using basic http rather than rest framework. Rest framework is a modern solution for building APIs with django. Rest framework provides the tools necessary to build a sustainable and performance-oriented API for Balancer.

Objectives:

1. Convert existing HTTP API views to class-based rest framework views.
2. Ensure API views are relevant to necessary functions of Balancer application.
3. Enhance and overhaul any outdated views.