Email reminder actual hours

The checkbox for the preference has been added and tested.
The mailer has been created.
The rake task has been set up.

All we need to do now is maybe test it locally (by someone other than me), deploy it, 
call the rake task by hand on a Monday and if it works as expected in production.

Once that's done we can simply configure the Scheduler to call it every day in the Dashboard as indicated on https://devcenter.heroku.com/articles/scheduler