Remove licensing hardcoded to services list

- Otherwise we'll end up with two in the services list when we've run the json migrations
Add an empty modules array

Migration script requires dashboards to have modules as a key within the
json
Merge pull request #803 from alphagov/add-empty-modules-array-to-licensing-stub

Add an empty modules array
Merge pull request #804 from alphagov/remove_hardcoded_licensing_from_dashboards

Remove licensing hardcoded to services list
Remove validation requirement for 1+ modules

It's possible to create a dashboard with no modules in the admin app,
so it should be possible here too.