Notifications Refactor — Refactor Legacy InboxStyle Notification

Description

We currently have updated notification styles and functions for sending notifications documented here. While we have moved the single message style to use the BigTextStyle, we haven't moved over the InboxStyle notifications yet. The older SummaryNotificationCreator class is still in use, and does not use the new InboxStyle described in our documentation. We should update it so the code matches what we claim we're using in the documentation, and to better maintain it in the future.

AC

• Create a feature flag for new notification style to test it during development and prior to deleting the old code
• Moved old inbox style notification sending to use new functions and style
• Fix any issues needed to ensure new InboxStyle works
• Update any tests
• Remove old inbox style notifications
• See if we can remove all legacy notification code