Stop defaulting "ids" to an empty array

This is so we can distinguish custom broadcasts in the Admin app
[1]. I've also extended the POST test for custom broadcasts to
check we're correctly reading data for "names", as this wasn't
being tested previously.

[1]: 411fda81c0

app/models.py

@@ -2338,7 +2338,6 @@ class BroadcastMessage(db.Model):
     def serialize(self):
         areas = dict(self.areas)
         areas["simple_polygons"] = areas.get("simple_polygons", [])
-        areas["ids"] = areas.get("ids", [])
 
         return {
             'id': str(self.id),