Delete customcert_pages rows after elements (#571)

Previously template->delete_page() deleted the corresponding
mdl_customcert_pages row, and then called element->delete() for each
element on the page.  But the latter then called
element_deleted::create_from_element() which relied on that
mdl_customcert_pages row to obtain the contextid for the event.

Now element->delete() is called for each element on the page, then the
mdl_customcert_pages row is deleted.

classes/template.php

@@ -215,11 +215,6 @@ public function delete_page($pageid) {
         // Get the page.
         $page = $DB->get_record('customcert_pages', array('id' => $pageid), '*', MUST_EXIST);
 
-        // Delete this page.
-        $DB->delete_records('customcert_pages', array('id' => $page->id));
-
-        \mod_customcert\event\page_deleted::create_from_page($page, $this)->trigger();
-
         // The element may have some extra tasks it needs to complete to completely delete itself.
         if ($elements = $DB->get_records('customcert_elements', array('pageid' => $page->id))) {
             foreach ($elements as $element) {
@@ -233,6 +228,11 @@ public function delete_page($pageid) {
             }
         }
 
+        // Delete this page.
+        $DB->delete_records('customcert_pages', array('id' => $page->id));
+
+        \mod_customcert\event\page_deleted::create_from_page($page, $this)->trigger();
+
         // Now we want to decrease the page number values of
         // the pages that are greater than the page we deleted.
         $sql = "UPDATE {customcert_pages}