Introduce new status, deprecate "out of scope"

The "not valid" status must have reasons why the need's not valid.
Migrate all "out of scope" statuses to "not valid"
Make sure need status is consistent when it's updated
Merge pull request #80 from alphagov/not-valid-status

Introduce the 'not valid' status
Add 'valid' need status
Add 'valid with conditions' need status
Need status: clear inconsistent fields before validation

This makes the model more consistent.
Extract need status strings into constants
Merge pull request #81 from alphagov/valid-statuses

Add Valid statuses