Reintroduce the ability to send timer reminders for arbitrary days

This now makes more sense as timer reminders are controlled by
individual days themselves rather than by user records.

app/interactors/send_timer_reminders.rb

@@ -1,10 +1,14 @@
 class SendTimerReminders
   include Interactor
 
+  before do
+    context.date ||= Date.current
+  end
+
   def call
-    User.for_timer_reminder.each do |user|
+    User.for_timer_reminder(date: context.date).each do |user|
       Notifier.timer_reminder(user).deliver_now
-      user.timer_reminder_sent!
+      user.timer_reminder_sent!(date: context.date)
     end
   end
 end