Add areas facet key

Allows searches by area identifier where no postcode is present. This allows the business-support-finder to lookup schemes by Imminence area slugs as well as postcode.
Depends on [this PR in Imminence](https://github.com/alphagov/imminence/pull/91) and [this PR](https://github.com/alphagov/publisher/pull/234) in Publisher.