Add setting to limit number of students subscribed to a course

[ywarnier]

Some portals open to the world are being abused by users registering more than a hundred users to courses, while the portal is only meant for academical use (which typically spans 15 to 30 students).

Add a configuration setting:

// Set a maximum number of users per course, globally. Set to 0 to remove the limit.
$_configuration['hosting_limit_users_per_course'] = 0;

To limit the number of users (including teachers) that can be subscribed to any course, globally.

[christianbeeznest]

It is added with this PR #6453

Thanks for confirmation.