Add facet for bfsf

Introduce a new *Business Size* facet for use in Question 3 of the Business Finance and Support Finder tool.

This question replaces a question about business type and therefore the branch *publisher > logic-change* should be merged at the same time as this branch.

The *Business Type* facet used by the replaced question has been left in the codebase for the time being but should be removed - a story will be added to the Mainstream (Development) Icebox for this.