Switch send_first_message to use background_queue
Switch send_first_message to use background_queue

It is not strictly just for reminders, but also
  - background_queue has better lag guarantees
  - staging does not have reminder_queue which makes the feature hard to test there on that queue

dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq