Add ability for admins to switch currency from dollars

[DevinWalker]

Details

Currently the US dollar is hard coded into the plugin code. We should provide admins the ability to change this currency to the one of their choosing.

Note: This is not a donor-facing currency switching capability.

Acceptance Criteria

• [x] New currency option is added to Gutenberg listing Stripe supported currencies
• [x] Currency displays on the frontend properly
• [x] Currency is formatted correctly
• [x] The donation is processed in the correct currency
• [x] Multiple donation forms on a page respect the chosen currency for the particular form