Upcoming activities due insight

Moodle is a passive LMS. This issue is about making it a bit more active by sending reminders to users about their upcoming due activities. This new feature require different modifications:

• A new analytics model based on assumptions
• Modifications to the insights system so models can override the default access control system (a capability check)
• A review of the action events implementations in core (calendar API) to confirm if they work specifying a different user than $USER

Most of the issues below are pending calendar API work to fully support passing a user instead of using $USER.