Update Popolo image urls

The included popolo json is being used as a definition of canonical
data and the image urls in it are expected to be fully qualified so
that they can be downloaded during import. Therefore, it makes sense
if they have the live Peoples Assembly url in them.

For #701
Merge branch 'issues/701-popolo-image-urls' of ssh://git.mysociety.org/data/git/public/pombola