Email reminders

This solves #241.
The platform will send emails to participants when the evaluation period of a course starts and it will send emails to due participants in case the evaluation period of any of their courses ends in 2 or 0 days (this is configurable in the settings).
could you look whether there is something better than using this method with 'due_participants'? here we know that all participants are due participants. maybe make a new method send_to_all_participants if that doesn't add too much code?