MDL-38196 Backup/restore: Display log if non-empty

When doing a backup or restore operation with the normal user interface,
if there is anything in the backup/restore log, it will now be displayed
at the end of the process below the Continue button.

If you have developer debugging enabled, it sets the display level to
LOG_DEBUG and there will always be content in the log. Otherwise, it
uses LOG_INFO which generally means the log is empty, and nothing
displays (no visible change for users).

backup/util/loggers/core_backup_html_logger.class.php

@@ -0,0 +1,46 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Logger that stores HTML log data in memory, ready for later display.
+ *
+ * @package core_backup
+ * @copyright 2013 The Open University
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class core_backup_html_logger extends base_logger {
+    /**
+     * @var string HTML output
+     */
+    protected $html = '';
+
+    protected function action($message, $level, $options = null) {
+        $prefix = $this->get_prefix($level, $options);
+        $depth = isset($options['depth']) ? $options['depth'] : 0;
+        $this->html .= $prefix . str_repeat('&nbsp;&nbsp;', $depth) .
+                s($message) . '<br/>' . PHP_EOL;
+        return true;
+    }
+
+    /**
+     * Gets the full HTML content of the log.
+     *
+     * @return string HTML content of log
+     */
+    public function get_html() {
+        return $this->html;
+    }
+}