Enable email notifications by default for course updates and post from instructor

[ayub02]

Current defaults were determined to account for 2U's email volume+cost. These are not optimal for everyone. However, once a notification preference is set as off, it's not easy to turn it on for all users.

I recommend that we turn EMAIL notifications ON by default with DAILY cadence for:

1. New post from instructor
2. Course update

See this slack msg for context: https://openedx.slack.com/archives/C057J2D1WU9/p1760366832399309

[AhtishamShahid]

PR : openedx/openedx-platform#37733