Add a warning on courses where start and end dates look wrong

Start and end dates are used for analytics and my overview (although I don't know exactly how in my overview) would be nice to have a process to display a warning in the main course page if the start and end dates look wrong.

The "look wrong" thing needs to be expanded and we need to think of ways to do it:

• no end date
• event dates out of start - end dates
• activities completion out of start - end dates
• course logs
• ...

This "look wrong" new method would also be used by prediction models where proper start and end dates are critical. At the moment the project inspire plugin has a course start and end date guess process https://github.com/moodlehq/moodle-tool_inspire/blob/master/cli/guess_course_start_and_end.php that should be using this new method as well.