EPIC: Enable notifications by default

The notification feature has been live on edx.org for 3 years now and can be considered stable. Therefore, to maximize the value of Open edX platform, we intend to make it default enabled in Verawood.
Learn more in the approach doc here: https://openedx.atlassian.net/wiki/spaces/OEPM/pages/5431164931/Approach+Making+notifications+tray+default+enabled."