Report days left more precisely

Rails's distance_of_time_in_words is disagreeably imprecise above 28
days, and saying "about 1 month" for 30 days isn't helpful in this
application.

Note: dividing the number of seconds by (26 * 60 * 60) to get days isn't
exactly correct in every scenario (e.g. summer/winter time and leap
seconds), but that doesn't matter here.