Improve i18n on shoppingcart

@dianakhuang, @sarina, can you two take a look at this?
@sarina: I was trying to figure out how best to handle the `<b>` nesting, and nested `ngettext` was the most straightforward thing I could think of. Do you think I should use `ngettext` for the "You have successfully purchased {items} for {course_names}." sentence, as well?