MDL-43033 cache: added a warning icon for mappings

We now show a warning icon with an explanatory alt if the user
has mapped a store as a mode default that doesn't not fulfill the
requirements of every known definition.

cache/admin.php

@@ -282,6 +282,7 @@ if (!empty($action) && confirm_sesskey()) {
 
 $PAGE->set_title($title);
 $PAGE->set_heading($SITE->fullname);
+/* @var core_cache_renderer $renderer */
 $renderer = $PAGE->get_renderer('core_cache');
 
 echo $renderer->header();