Convert business support areas to slugs

Depends on [this PR](https://github.com/alphagov/imminence/pull/91)
Bring areas data in line with the expectations of the business support finder and consistent with business support facet values.

- Migrates existing Imminence area identifiers to slugs.
- Converts the business support editiorial form to expect slugs for areas.

https://www.pivotaltracker.com/story/show/74594086