Remove unused Need#in_scope field

It's no longer used in Maslow.
Remove usused Need#out_of_scope_reason field

Maslow now uses Need#status#reason for this information.
Command for changing organisation slug
Add rake task to change org slug
Define equality for an IndexableNeed as value equality

This is a convenience to help with testing.
Ensure organisation `_id` field properly updated

Need to create a new organisation and destroy the old one, so that the
_id field is updated. The `_id` is expected to match the `slug`.
Update associated needs of a changed organisation

- change the organisation_ids reference
- re-index the need in search
Make need status mandatory

If the status is missing, then it defaults to "proposed".
Merge pull request #77 from alphagov/make-status-mandatory

Make need status mandatory
Merge pull request #78 from alphagov/remove-in_scope-out_of_scope_reason

Remove Need#in_scope and Need#out_of_scope_reason
Record need change using appropriate user

This means that if someone looks at the change history of the need
it will be clearer why it was changed
Merge pull request #79 from alphagov/rake-task-to-change-org-slug

Rake task to change org slug
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