New TurnKey Drupal 8 version (15.3)

Changes:

• Includes latest upstream Drupal8 - v8.6.3.
• Fix escaping for inithook inputs to support complex passwords - closes #1247 [ Stefan Davis ]
• Rebuild to resolve inadvertent removal of mariadb during sec-updates
  • part of #1246.

Links

• Release meta-files (signature, manifest)