Debt free day integration

These are the changes required to make the DebtFreeDayCalculators run inside the frontend application.

## Instructions to Run

2. Clone `frontend` and checkout the `debt_free_day_integration` branch
3. Run the usual install instructions for frontend
4. Create a `features.yml` file to enable the engine (see below)
5. Run the frontend application
6. Visit `http://localhost:5000/en/tools/loan-calculator` and `http://localhost:5000/en/tools/credit-card-calculator`

features.yml:
```
features:
  debt_free_day_calculator: true
```