Add new field "applies_to_all_organisations"

As we add "generic" needs which apply to all organisations, we need a more elegant way of indicating that they apply to all organisations rather than just assigning every organisation to the need.

This adds a new boolean field to highlight "generic" needs. The word "generic" doesn't really mean much, so "applies to all organisations" makes more sense.

This branch:
- adds the new field to the `Need` model
- exposes the new field in the API in both need presenters
- indexes the new field in search.