MDL-56705 forms: Use [] for multiple values

Some "select" based form elements are not inheriting directly from
MoodleQuickForm_select - so did not have the adjustment to the form
field name that allows multiple values. I moved this fix to the
templateable trait so all the form fields get this fix.

lib/form/select.php

@@ -224,10 +224,6 @@ class MoodleQuickForm_select extends HTML_QuickForm_select implements templatabl
         }
         $context['options'] = $options;
 
-        if ($this->getAttribute('multiple')) {
-            $context['name'] = $context['name'] . '[]';
-        }
-
         return $context;
     }
 }

lib/form/templatable_form_element.php

@@ -82,6 +82,11 @@ trait templatable_form_element {
         $context['type'] = $this->getType();
         $context['attributes'] = implode(' ', $otherattributes);
 
+        // Elements with multiple values need array syntax.
+        if ($this->getAttribute('multiple')) {
+            $context['name'] = $context['name'] . '[]';
+        }
+
         return $context;
     }
 }