Show when a need applies to all organisations

When a need applies to all organisations, highlight this with a message
on the need view page and when browsing the list of needs.
Default the organisation_ids field to be an empty array

When the organisation_ids field is nil, set it to be an empty array so
that it's consistent with the other array fields we send to the Need API
like 'justifications' and 'met_when'.
Hide the organisations field for a generic need

Don't show the organisations field for a need which applies to all
organisations, as it will be rejected by the Need API.
Merge remote-tracking branch 'origin/master' into generic-needs
Display a message in the form for a generic need

When we omit the organisations field for the form, show a message
in place so that users aren't confused by the missing field.
Merge pull request #51 from alphagov/generic-needs

Add some custom behaviour for 'generic' needs