Refactor business support api helper

Make the business support api helper behave like the API.
The previous method of stubbing and retrieving a stub depended on
an exact match of facet keys and values. The API searches facet values
based on an $in clause. See https://github.com/alphagov/govuk_content_models/blob/master/app/models/business_support_edition.rb#L67
Also sanitises stub facets for backwards compatibility.
Merge pull request #153 from alphagov/update_business_support_helpers

Refactor business support api helper