VPN Prices should be formatted for the locale of the page

[pmac]

We're currently getting this almost right, but in some cases the format of the number and symbol (e.g. 4.99 € vs €4,99) vary quite a bit for different locales. As part of this we should move to letting Fluent handle this kind of formatting.

FluentNumber is a class in the Fluent system that can format number input for a particular locale and currency. It would be nice if we could build in handling for currency formatting into the ftl() helper in bedrock for use in templates.

[alexgibson]

I opened a PR linked above for the short term fix that updates Euro formatting for en locales.

Looking in bedrock, we're already using babel's format_currency helper in some places for the newer Relay code. We could use that for VPN also, however if we want to add currency formatting to the ftl() helper like you suggest, I'll leave that for a back-end dev to take a look at.