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).

theme/bootstrapbase/less/moodle/backup-restore.less

@@ -217,3 +217,15 @@
 .path-backup .wibbler .state12 {
     background: #222;
 }
+.path-backup .backup_log {
+    margin-top: 2em;
+}
+.path-backup .backup_log h2 {
+    font-size: 1em;
+}
+.path-backup .backup_log_contents {
+    border: 1px solid #ddd;
+    padding: 10px;
+    height: 300px;
+    overflow-y: scroll;
+}