Add some custom behaviour for 'generic' needs

**This should not be merged before #49, as this pull request branches from that one.**

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This changes Maslow to handle needs which are "generic" across government organisations (those with the `applies_to_all_organisations` field set to true) using custom behaviour.

Specifically, "generic" needs:
- show a message instead of comma-separated organisations in the search results and needs list
- have an information callout added to the single need view to highlight that it is a "generic" need
- cannot have organisations added in the need edit form (this is consistent with the validation constraint in the Need API which prevents organisations being added for "generic" needs).