Currency formatting for payment details in Thank You View

Previously, PaymentSource's unicode representation was used to display
the payment details. This had several problems, e.g. currency formatting
wasn't working as reported in #972.

It's been replaced by a port of the unicode function to Django's
template language. This should make customisation a lot easier, and
currency formatting should work as expected.

Fixes #972
Bump version of detox

The previous version stopped working on my machine. Merely upgrading to
the latest version fixes the issue. I presume some other non-pinned
dependency introduced a breaking change.