This reverts commit 6601690202.
This commit was adding the 'returnurl' param unnecessarily to the
course links in the navigation block, causing a lot of long URLs
to be created.
This patch improves usability of enrolment page in case of course is not
enrollable. 'Continue' button now returns student to referring page instead of
main moodle page (as it used to be). To make this improvement, passing correct
returnurl parameter to enrol/index.php page was implemented for links that may
be accessible for not-enrolled students.
This adds a category expanded which:
* fetches child content in a category tree if it has not already been loaded;
* toggles relevant classes on the category node to show and hide child content; and
* applies appropriate animations to improve user experience.