Add 'valid' and 'valid with conditions' statuses

https://trello.com/c/Og7bZ843/23-enable-needs-validation-in-maslow

This feature enables reviewers of user needs to mark needs as valid or valid only if they satisfy certain conditions.

![image](https://cloud.githubusercontent.com/assets/23801/5296297/273a33f6-7b9a-11e4-9285-7ed795c465d9.png)

![image](https://cloud.githubusercontent.com/assets/23801/5296317/4e75e0b4-7b9a-11e4-85a4-fa3e3eefad05.png)

![image](https://cloud.githubusercontent.com/assets/23801/5296289/192d0fd6-7b9a-11e4-8583-232eb479e361.png)

![image](https://cloud.githubusercontent.com/assets/23801/5296313/447564ea-7b9a-11e4-90a2-c2313d2a699e.png)

Note: https://github.com/alphagov/govuk_need_api/pull/81 needs to be merged first
there are various errors being populated in flash, so we should assert on the exact message, or else we're not asserting what path is getting executed.