Even more currencies

[Changaco]

#1286 increased the number of currencies supported by Liberapay from 2 to 33. This is great, but Stripe supports 135+ presentment currencies, so we can still improve.

The reason why we support 33 currencies is because we use the daily exchange rates published by the ECB, and that list only contains 32 currencies, so what we need is another source of exchange rates. We're not interested in real-time rates, daily rates are better for our use case.

Edit: Open Collective uses https://fixer.io/.