MDL-63485 core_user: Allow filtering by No groups on participants page

A new optional parameter $context has been added to the
core_group::groups_get_members_join() function.
Besides, some core_group methods now accept -1 (USERSWITHOUTHGROUP) for
the groupid field.

Backport of MDL-61967.

user/renderer.php

@@ -145,6 +145,11 @@ class core_user_renderer extends plugin_renderer_base {
         if (has_capability('moodle/site:accessallgroups', $context) || $course->groupmode != SEPARATEGROUPS) {
             // List all groups if the user can access all groups, or we are in visible group mode or no groups mode.
             $groups = $manager->get_all_groups();
+            if (!empty($groups)) {
+                // Add 'No group' option, to enable filtering users without any group.
+                $nogroup[USERSWITHOUTGROUP] = (object)['name' => get_string('nogroup', 'group')];
+                $groups = $nogroup + $groups;
+            }
         } else {
             // Otherwise, just list the groups the user belongs to.
             $groups = groups_get_all_groups($course->id, $USER->id);