Add an option for falling back to a default locale for public body listings

This branch is intended as a fix for #1000. It seems that there are really two different behaviours a site implementor might want here - either to have public body listings matching only from the current locale, or to additionally also include those matching in the default locale.  (The old behaviour was neither of these: it was to only match in the default locale, which seems less useful than the latter.)  So, this commit introduces an option to also include public body listings from the default locale as a fallback (defaulting to `false`).