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
That's fair feedback. I've changed the label to `This need is valid under the following conditions: <conditions>` in 3f76d00.