User external

• use user_external plugin instead of user_backend_sql_raw (fixes #51)
• creates config from template instead of writing it directly from postinstall.sh (easier to understand)
• remove nextcloud database init sql files (not needed anymore)
• fixes nextcloud install (env vars dont work with maintenance:install)
• creates nextcloud db with utf8mb4 instead of utf16

TODO:

• Nextcloud is using sqlite as backend (should be fixed with --database param)
• we still need to run filecache db migrations like before (should be fixed)
• nextcloud replaces trusted_domains with localhost during installation
• user signup via welcome.$DOMAIN is not working
• rebase on top of !43 (merged)
• rebase postfixadmin on master to fix error during installation (Error: password_expiry must be numeric when running postfixadmin-cli domain add)
• password reset email not sent (some problem with ssl in postfixadmin)
• make upstream PR for postfixadmin ssl sendmail (link)
• rebase on master
• merge docker-create-account!2 (merged), release it and use that
• merge docker-postfixadmin!4 (merged), release it and use that

testing:

• make a fresh installation from this branch (run bash bootstrap-generic.sh https://gitlab.e.foundation/e/priv/infra/compose user-external)
• create a new account with uppercase and lowercase characters
• login to Nextcloud with username/domain in lowercase
• login to Nextcloud with username/domain in uppercase
• click on the password reset link in Nextcloud, and reset your password
• ensure that you can login to Nextcloud with the new password
• change your email in Nextcloud, and make sure you see the new address in postfixadmin (mail.$DOMAIN) (you can execute scripts/sync-emails.sh manually to avoid waiting)