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/views/explore/posts/popular.html.erb

@@ -11,7 +11,7 @@
       <% if @scale == "day" %>
         <%= @date.strftime("%B %d, %Y") %>
       <% elsif @scale == "week" %>
-        <%= @date.strftime("Week of %B %d, %Y") %>
+        <%= @min_date.strftime("%B %d, %Y") %> - <%= @max_date.strftime("%B %d, %Y") %>
       <% elsif @scale == "month" %>
         <%= @date.strftime("%B %Y") %>
       <% end %>