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 
Cool. I made it just one on `deleted_at`, I don't think the `created_at` one was ever used.

```
mysql> describe select * from deploys order by created_at desc, id desc;
+----+-------------+---------+------+---------------+------+---------+------+-------+----------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows  | Extra          |
+----+-------------+---------+------+---------------+------+---------+------+-------+----------------+
|  1 | SIMPLE      | deploys | ALL  | NULL          | NULL | NULL    | NULL | ? | Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+-------+----------------+
1 row in set (0.00 sec)

mysql> describe select * from deploys order by created_at desc;
+----+-------------+---------+------+---------------+------+---------+------+-------+----------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows  | Extra          |
+----+-------------+---------+------+---------------+------+---------+------+-------+----------------+
|  1 | SIMPLE      | deploys | ALL  | NULL          | NULL | NULL    | NULL | ? | Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+-------+----------------+
1 row in set (0.00 sec)
```