Make sure no SayIt Speaker is associated with a blacklisted PopIt IDs

Even if a PopIt ID was blacklisted, a speech might still end up
being associated with that person because there would still be a
Speaker object with the exact name that's being resolved which
has its person attribute set to a Person with the blacklisted
PopIt ID.

This commit makes sure that any Speaker with a person attribute
with a blacklisted PopIt ID has that person attribute set to None
when the ImportJson object is initialized.

Fixes mysociety/pombola#1740

za_hansard/importers/import_base.py

@@ -14,6 +14,10 @@ def __init__(self, instance=None, commit=True, popit_url=None, popit_id_blacklis
         self.ai, _ = ApiInstance.objects.get_or_create(url=popit_url)
         self.person_cache = {}
         self.popit_id_blacklist = set(popit_id_blacklist or ())
+        # Make sure that there are no speakers associated with
+        # blacklisted PopIt IDs:
+        Speaker.objects.filter(person__popit_id__in=self.popit_id_blacklist) \
+            .update(person=None)
 
     def set_resolver_for_date(self, date_string='', date=None):
         self.resolver = ResolvePopitName(date=date, date_string=date_string)