Deleting and updating tickets

Tickets can be deleted by sending a request to `/api/ticket/:ticketId/delete`. Only global admins and admins where the ticket was created in can delete tickets.

There is currently no endpoint for updating tickets, since this is handled by ZenDesk. When a ZenDesk ticket gets updated, this will be synced with our tickets. Therefore, a public method has been created (`updateTicket`) which accepts a `ticketId` and an `opts` object. The `opts` object contains `status` and `lastModified`. All other parameters that are passed in will be stripped from the `opts` object.