Create calculator React app

[elvismdev]

AC:

• Use this React currency calculator code as starter. Port it into the React app sitting in /calculator page. (Video about this code: https://www.youtube.com/watch?v=XN5elYWiSuw)
• No need to use an API call. Use the exchange rate value passed on page load. (https://github.com/elvismdev/usdcup.io/issues/94)

Technical Notes:

• A new /convert page already exists in the dev branch that renders an starter React app. Route defined here: https://github.com/elvismdev/usdcup.io/blob/dev/src/Controller/CurrencyConverterController.php#L15
• The React app contains a avgExchangeRate variable with value ready to be used for required calculations of the currency converter. Defined here: https://github.com/elvismdev/usdcup.io/blob/dev/assets/components/currency_converter/CurrencyConverter.js#L6-L8

[elvismdev]

Handled in https://github.com/elvismdev/usdcup.io/pull/100