Update core_send_people_to_popit for the current PopIt API

The core_send_people_to_popit hadn't been used for some time, and only
worked with a very old version of the PopIt API. These commits update it so
that this command is useful again. For example, a PopIt instance
corresponding to the current South Africa site can be found here:

   http://za-new-import.popit.mysociety.org/api/v0.1/