popular posts: fix date range handling.

* Fix the next button for the weekly timescale to jump to the next week,
  not the next day.
* Show the start and end dates for the weekly timescale.
* Use `Time.zone.today` instead of `Date.today` to respect the user's
  timezone setting.

app/helpers/popular_posts_helper.rb

@@ -4,7 +4,7 @@ module PopularPostsHelper
     when "day"
       date + 1.day
     when "week"
-      date + 1.day
+      date + 1.week
     when "month"
       1.month.since(date)
     end