use two decimals in input in giving knob

[chadwhitacre]

Raised on Twitter (cc: @JVimes). The issue is that we're using format_decimal there, and it doesn't know how to enforce two decimal places.

I think there may be other places we're afflicted by this as well? Maybe re: widgets?

[chadwhitacre]

template: https://github.com/gratipay/gratipay.com/blob/1942/templates/your-payment.html#L22

format_decimal: https://github.com/gratipay/gratipay.com/blob/1942/gratipay/utils/i18n.py#L219 (delegated to babel)

[mattbk]

Maybe re: widgets?

I think I solved this in gratipay/grtp.co#118, but that's JS...

[aandis]

python-babel/babel#90

[etanol]

I've seen this issue linked from python-babel/babel#90. You need to keep in mind that format_currency and format_decimal have different number patterns by default. Since you can specify an explicit number pattern, my advice is to to the following:

1. Extract the default currency pattern of the locale in use
2. Remove the currency symbols from it: ¤
3. Use the result as an explicit pattern to either format_decimal or format_currency