Fix MPL party memberships

As explained in #1153 by @paullenz, @geoffkilpin and @mhl, the ZA party memberships don't show up on some pages because they have a PositionTitle of "Party Member" not plain "Member". This pull request provides a new management command to fix this up:

```
./manage.py south_africa_fix_position_titles
```

And also tidies up the source data in south-africa-popolo.json to name every "Party Member" role "Member" instead.

Closes #1153