Load currency dictionary from .json into both: backend model and frontend form.

[Reckless-Satoshi]

List of available currencies are a hack at the moment.

Would be great if the list of currencies could live in the static folder as a .json and be read from there by both:

• The choices field of the Order model
  https://github.com/Reckless-Satoshi/robosats/blob/9335e6abbe797e4fb281d7280da3a7114c7ef03b/api/models.py#L18-L21

• The Select form in the front en MakerPage
  https://github.com/Reckless-Satoshi/robosats/blob/9335e6abbe797e4fb281d7280da3a7114c7ef03b/frontend/src/components/MakerPage.js#L139-L150

• Create preliminary .json file with currency number and currency code

• Replace Currency IntegerChoices for the options in the currencies.json in models.py

• Replace Select currency options for the options in the currencies.json in MakerPage.js

• Update the logics of handleCurrencyChange to print the right currency code over the "Create Order" button.