Send alert chord has tasks with ignore_result = True

While investigating #480 (closed), I discovered that there are tasks in the canvas that sends alerts that have ignore_result=True. According to the celery documentation,

Chaining a group together with another task will automatically upgrade it to be a chord:

and

Tasks used within a chord must not ignore their results.

This isn't causing any obvious problems as far as I can tell, but there are some quirks in the workflow that may be related (including #480 (closed)). Even if not, it sounds like our canvas doesn't have well defined behavior, so we should fix it to avoid unintended consequences in the future.