Feature/sla

This pull adds a "Revolving" option to SLAs that will base the ticket overdue trigger on the ticket.lastresponse date instead of ticket.created or ticket.reopened. It currently will not mark tickets as overdue if they are answered and waiting on a response from the requester. 

I thought I had the DB patch configured correctly but the admin dashboard states "The upgrader does NOT support upgrading from the current patch [6de40a4d5bad7a2923e769a4db1ff3b9]!". I am submitting it anyway so that it can be reviewed and hopefully someone can point out what, if anything, I have done wrong patching the DB.