Restrict access to course based on start and end date

When this issue was created, it was not possible to specify a starting/end date in the course settings. 

When these settings were added, they were only used for calendar purposes (for instance, to display the course in the Past/Present/Future course list). However, this is confusing because some users were expecting this date to affect the course visibility (which is a separate setting).

This issue adds a couple of scheduled tasks to change the course visibility automatically when the start/end course date matches the current date.

These tasks will be disabled by default, to keep the current behaviour. Once they are enabled, only courses with start/end dates near the past 24 hours will be checked. The visibility of these courses will only be changed when their start/end dates are higher than the current one, to avoid updating the course visibility early.

The tasks are based on the code of the https://moodle.org/plugins/local_culcourse_visibility plugin maintained by Amanda Doughty, although the following changes have been applied:

• Instead of having one single task, it has been split into 2 different tasks (that way, admins will be able to decide which ones they want to enable/disable).
• A couple of events, course_started and course_ended have been created and are raised when a course visibility is automatically changed.
• Only courses with start/end dates near the past 24 hours will be checked, but they are only changed if the current date is higher than the one defined in the course settings (the original task was changing any course with the current date, ignoring the hour).
• A different help message will be displayed for start/end date settings when these tasks are enabled.