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.

group/templates/comboboxsearch/group_selector.mustache

@@ -34,10 +34,10 @@
 <span class="d-none" data-region="groupid" data-groupid="{{group}}"></span>
 <div class="align-items-center d-flex">
     <div class="d-block pe-3 text-truncate">
-        <span class="d-block small" aria-hidden="true">
+        <label class="d-block m-0 small" aria-hidden="true">
             {{label}}
-        </span>
-        <span class="p-0 font-weight-bold">
+        </label>
+        <span class="p-0 font-weight-bold" data-selected-option>
             {{selectedgroup}}
         </span>
     </div>