Add indices for slow queries / to account for deleted_at

I think there are a bunch here we can actually discuss. The most necessary ones are probably on `jobs.status` and `deploys.{stage_id,deleted_at}` (for DeploysController#active).

@zendesk/samson @zendesk/octo 
That's the one I wasn't sure about. The default scope is 'where deleted_at IS NULL', 'order by created_at DESC' and it appears the mysql will only use an index for both parts if the order is 'where key1 order by key2' (https://dev.mysql.com/doc/refman/5.0/en/order-by-optimization.html)

https://github.com/zendesk/samson/blob/master/app/models/deploy.rb#L2
https://github.com/zendesk/samson/blob/master/app/models/deploy.rb#L8