Add a new in_scope field so that Needs can be marked as out of scope

This adds a new boolean field to Need, `in_scope`, which is used to indicate whether a need is in scope for GOV.UK.

Right now, we only want to be able to mark a need as out of scope, as there will be future work to add a workflow for deciding that a need should be in scope on GOV.UK. To reflect this, the `in_scope` field can only be `nil` or `false` at the present time.

The `in_scope` field is indexed in search so that it is returned as part of the need list view.