Migrate 8.x branches to separate git repo

Background: The civicrm-drupal.git repo has branches named 8.x-*. However, these are not detectable/addressable via composer/packagist.

Tasks

(a): Create new repo civicrm-drupal-8. Migrate the 8.x-* branches, but remove the 8.x- prefix.
(b): Update release process to ensure that these are branched/tagged.

Edited Apr 19, 2018 by totten

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