Course instance is available to students immediately upon creation

[jonatanschroeder]

Once a course is requested and the template is used as a basis for a new course instance, the course instance becomes available immediately in the enrollment page, even before the instructor has a chance to do anything about it. This may cause confusion with students enrolling in a course too early. Some suggestions:

• Phase out the enrollment page (Improve enrollment management PrairieLearn#9217);
• Change the template starting dates to something no longer current, or very far in the future;
• Change the template to include "hideInEnrollPage": true by default;
• Include some information in the course request page such as a starting date that is then updated in the course initialization process (not a huge fan of this option, adding for completeness).