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)

The disabled state here is not _really_ disabled. Clicks are prevented using `pointer-events: none;`, which will only work in modern browsers. And if using keyboard nav the modal will still appear when pressing enter.

Does the disabled button need to be here at all?
Should there be an explanation why the need can't be validated?