SLA for services

This change adds SLA functionality for services configured in Selena. This new functionality takes service history of each active service and counts SLA. Calculated SLA is stored in DB for every day. The data are displayed on a front page. There are 3 information available: SLA for 7 days, 1 month and 3 months. 
can you add verbose_names in Meta and also add some translation to the fields? 
https://docs.djangoproject.com/en/dev/topics/i18n/translation/#model-fields-and-relationships-verbose-name-and-help-text-option-values
Even if it's not there in other things, we'd like to bring it in line sometime...