JSON export: also export data usable by mongoimport without --jsonArray

When importing the Popolo JSON with mongoimport's --jsonArray, there's a
size limit that we keep hitting.  There's no such limit if you use the
format generated natively by mongoexport, which is one JSON object per
line.  This commit changes core_export_to_popolo_json so that it also
outputs this format.

Fixes #1447