Use .dump an extension for files for mongoimport

This is consistent with a change to replace-database - the
one-JSON-object-per-line format for mongoimport isn't valid JSON, so
call it mongo-COLLECTION.dump instead.
Popolo JSON export: join base URL and image URL more robustly

If you included a trailing slash in the base URL, and the image URL began
with a slash then you'd end up with a doubled slash and the image would
be 404; by using urljoin to join them this will be avoided.