Remove User#at_midnight

Replace it with a class called DateUtils with a method called
`#midnight_for` that takes a preference instance and date to convert to
midnight.

lib/date_utils.rb

@@ -0,0 +1,5 @@
+class DateUtils
+  def self.midnight_for(prefs, date)
+    ActiveSupport::TimeZone[prefs.time_zone].local(date.year, date.month, date.day, 0, 0, 0)
+  end
+end