MDL-18309 Course: enrol/index.php returnurl improvement

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.

course/renderer.php

@@ -49,6 +49,9 @@ class core_course_renderer extends plugin_renderer_base {
      */
     protected $strings;
 
+    /** @var moodle_url|null stores page where to return to from the enrolment page */
+    public $returnurl;
+
     /**
      * Override the constructor so that we can initialise the string cache
      *
@@ -1207,7 +1210,8 @@ class core_course_renderer extends plugin_renderer_base {
 
         // course name
         $coursename = $chelper->get_course_formatted_name($course);
-        $coursenamelink = html_writer::link(new moodle_url('/course/view.php', array('id' => $course->id)),
+        $coursenamelink = html_writer::link(new moodle_url('/course/view.php',
+                                                           array('id' => $course->id, 'returnurl' => $this->returnurl)),
                                             $coursename, array('class' => $course->visible ? '' : 'dimmed'));
         $content .= html_writer::tag($nametag, $coursenamelink, array('class' => 'coursename'));
         // If we display course in collapsed form but the course has summary or course contacts, display the link to the info page.