Implement D8 initialization for civicrm-setup

Background: When installing on Drupal 8, settings such as DSN and locale should be autodetected. Task:

Copy plugins/init/Drupal.civi-setup.php or plugins/init/WordPress.civi-setup.php
Create Drupal8.civi-setup.php
Adapt for D8. For source material on how to detect things like DSN, look at civicrm-drupal@8.x-master in civicrm.install / drush.civicrm.inc.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information