Update environment variables for readability

## Goal
- The env variables' names on the frontend and the backend are more understandable 

## Deadline / Task size
- 0.5 day

## Steps
- Change `URL` to `MONGODB_URL` on the backend and  `BACKEND_URL` on the frontend

## Uncertainty
- Are there name conventions specifically for MongoDB's URL and backend URLs?
  - Still researching

## Additional memo
- None 