Replace marking needs as 'out of scope' with 'validation decision' recording

This is the first step in allowing needs to be validated. A newly created need is `proposed`, and this change allows to mark the need as `not valid` with reasons why it's not valid. Subsequent PRs will add the `valid` and `valid with conditions` states.

**Note**: A corresponding Need API is needed, so https://github.com/alphagov/govuk_need_api/pull/80 must be merged first for this PR to work.

![image](https://cloud.githubusercontent.com/assets/23801/5229083/d14e5f6e-770a-11e4-98e5-3a9d646add91.png)
![image](https://cloud.githubusercontent.com/assets/23801/5229102/fed5d7aa-770a-11e4-9ecb-28807c094082.png)
![image](https://cloud.githubusercontent.com/assets/23801/5229108/06d5e3aa-770b-11e4-8ad3-a24c92b154e9.png)
![image](https://cloud.githubusercontent.com/assets/23801/5229133/4ddb2a1c-770b-11e4-86a9-c17bcafb6640.png)

## Non-JS version

![image](https://cloud.githubusercontent.com/assets/23801/5229116/19857ae2-770b-11e4-95a8-ab6bcd3b5a49.png)

"Decide on need"?