feat: redone the structure of the app for better & easy maintainability

Changes

Refactoring includes:

1. Added flask-restx as a package to transform routes into resources.
2. Used the DAO Pattern for Subscriptions with as much inheritance as possible to make the code easy to organise & maintain.
3. Fixed the startup issues with the clumsy import pattern.
4. Created 3 different layers of endpoints, Internal API's, External API's & Template Routes.
5. All the dashboard templates call the Internal API's for data. This makes the abstraction between frontend & backend possible.
6. Added scripts for running the backend & the mock server in the pipenv.

Notes

1. Signup & Login Flow is working. There might be some error-handling left, but with the new structure that should be easy to add.
2. Creating, Modifying & Deleting subscriptions is in works, and should be done really soon.
3. Celery with RabbitMQ for cron scripts is being tested locally, and will be added subsequently.