move slow sms provider threshold from 10% to 20%

provider switching is a process that can happen as often as we like
without disrupting the flow of the system - however, there are some
reasons why we might not want to switch. One problem we've seen is
when a provider is having an issue, we might switch away from them
manually only for the app to automatically switch back to them again
and again.

Long term we'd like to have a system better suited for sharing the load
equally between our two sms providers, but short term, by increasing
the threshold for switching from 10% (of messages sent are slow) to
20%, we hope to make switching happen less often.

A notification is considered slow if it was sent in the last ten
minutes, on the current provider, and is either

* still in sending or pending after 4 minutes
* in delivered, but took at least 4 minutes to send

app/celery/scheduled_tasks.py

@@ -106,7 +106,7 @@ def switch_current_sms_provider_on_slow_delivery():
         return
     slow_delivery_notifications = is_delivery_slow_for_provider(
         provider=current_provider.identifier,
-        threshold=0.1,
+        threshold=0.2,
         created_at=datetime.utcnow() - timedelta(minutes=10),
         delivery_time=timedelta(minutes=4),
     )