Output 'primary' as a boolean if formatversion=2

Adds the field as a true boolean in all cases, rather than conditionally
as an empty string if the coordinates are primary for the article.

These are automatically converted back to "version 1" output for clients
requesting that JSON version, per [1].

[1] https://www.mediawiki.org/wiki/API:JSON_version_2#API_module_implementers:_ensuring_backwards_compatibility

Bug: T177428
Change-Id: Ia2e81f8aa060a69095a8e850569cc741c791973a