MDL-26854 COMMENT

1. added pluginname_comment_validate callback to comments api
2. change permission callback defaults to false
3. Tidied up serveal areas of comments and implemented callback to
ignore system permissions for view (credits to Sam Hemelryk)

AMOS BEGIN
    MOV [modulerejectcomment,error],[callbackrejectcomment,error]
AMOS END

comment/index.php

@@ -20,7 +20,7 @@
  */
 require_once('../config.php');
 require_once($CFG->libdir.'/adminlib.php');
-require_once('locallib.php');
+require_once($CFG->dirroot.'/comment/locallib.php');
 
 require_login();
 admin_externalpage_setup('comments', '', null, '', array('pagelayout'=>'report'));
@@ -81,8 +81,11 @@ if (!empty($err)) {
 }
 if (empty($action)) {
     echo '<form method="post">';
-    $manager->print_comments($page);
-    echo '<input type="submit" id="comments_delete" name="batchdelete" value="'.get_string('delete').'" />';
+    $return = $manager->print_comments($page);
+    // if no comments available, $return will be false
+    if ($return) {
+        echo '<input type="submit" id="comments_delete" name="batchdelete" value="'.get_string('delete').'" />';
+    }
     echo '</form>';
 }