MDL-41838 Backup/restore: Support .tar.gz format for .mbz (2 of 2)

The new experimental setting enabletgzbackups allows backups to be
created so that the internal format for .mbz files is .tar.gz.

Restore transparently supports .mbz files with either internal
formats (.zip or .tar.gz).

The .tar.gz format has the following benefits for backup:
- Supports larger files (no limit on total size, 8GB on single file
  vs. 4GB limit on total size)
- Compresses text better, resulting in smaller .mbz files.
- Reports progress regularly during compression of single files,
  reducing the chance of timeouts during backups that include a
  very large file.

Time performance may also be improved although I haven't done a
direct comparison.

admin/settings/development.php

@@ -19,6 +19,15 @@ if ($hassiteconfig) { // speedup for non-admins, add all caps used on this page
     $enablecssoptimiser->set_updatedcallback('theme_reset_all_caches');
     $temp->add($enablecssoptimiser);
 
+    // Backup archive .mbz format: switching to .tar.gz enables larger files, better
+    // progress reporting and possibly better performance. This is an experimental
+    // setting but if successful, should be removed and enabled by default in a future
+    // version. Note: this setting controls newly-created backups only; restore always
+    // supports both formats.
+    $temp->add(new admin_setting_configcheckbox('enabletgzbackups',
+            new lang_string('enabletgzbackups', 'admin'),
+            new lang_string('enabletgzbackups_desc', 'admin'), 0));
+
     $ADMIN->add('experimental', $temp);
 
     // "debugging" settingpage