Use django_crontab for sending notification on end of month.

[DavidKozina]

• Uses django_crontab, required Cron
• Links to session page does not work yet
• Cronjob runs everyday at 00:00 and checks assigns
• It sends notification 2 workdays before end of month if user have unassigned hours that month
• It sends notification everyday in new month until user assign all hours in last month
• Development: We could use django_crontab for other cool stuffs

Signed-off-by: DavidKozina david.kozina@volny.cz