MDL-80746 core_course: fix group_selector rendering

This change:
- Fixes the group_selector constructor, removing the unnecessarily
deprecated param (this code is main-only, so no need to deprecate) and
fixes all calling code.
- moves the button and content into separate named_templatable
renderables, cleaning up the group_selector code so that it only needs
to make a single call to render and doesn't concern itself with contexts
of other renderables.

mod/assign/classes/output/grading_actionmenu.php

@@ -136,7 +136,7 @@ class grading_actionmenu implements templatable, renderable {
 
         if (groups_get_activity_groupmode($cm, $course)) {
             $data['groupselector'] = $actionbarrenderer->render(
-                new \core_course\output\actionbar\group_selector(null, $PAGE->context));
+                new \core_course\output\actionbar\group_selector($PAGE->context));
         }
 
         if ($extrafiltersdropdown = $this->get_extra_filters_dropdown()) {