Fix setting of broadcast permission

This was broken because current_service doesn’t update itself after
calling the `update` method of the API. So we thought we were changing
the permissions like this:
```
{'email', 'sms', 'letter'}
{'email', 'sms', 'letter', 'broadcast'}
{'sms', 'letter', 'broadcast'}
{'letter', 'broadcast'}
{'broadcast'}
```

But actually we were doing this:
```
{'email', 'sms', 'letter'}
{'email', 'sms', 'letter', 'broadcast'}
{'sms', 'letter'}
{'email', 'letter'}
{'email', 'sms'}
```

This commit changes the code to update the permissions like this:
```
{'email', 'sms', 'letter'}
{'broadcast'}
```

It does so by adding a new method to the service model which changes all
the permissions in one API call, and updates the tests to mock the
underlying API call, not the method on the model.

app/main/views/service_settings.py

@@ -318,10 +318,7 @@ def service_set_broadcast_permission(service_id):
     if form.validate_on_submit():
 
         if form.enabled.data:
-            current_service.force_permission('broadcast', on=True)
-            current_service.force_permission('email', on=False)
-            current_service.force_permission('sms', on=False)
-            current_service.force_permission('letter', on=False)
+            current_service.force_broadcast_permission_on()
         else:
             current_service.force_permission('broadcast', on=False)