Use standard I18n for currency and dates

My main goal was to add year to printed dates, but I ended up with this.
Rename purchased_at -> purchased_on

The column is a `Date`, so it should have the `_on` suffix. The `_at` suffix is
reserved for `DateTime`s.
Merge pull request #114 from datasektionen/vk-timestamp-year

I18n for currency and dates, and rename purchased_at -> purchased_on