Convert the rest of this configuration file to slapos.

It is straightforward conversion of parameters except:

    - access-via-ssh is disabled (gitlab slapos version does not support
      ssh access and supports HTTP(S) only by design on purpose)

    - we do not support restricting possible projects visibility via
      instance parameter (very low chance this will be needed in
      practice)

    - default issue-closing pattern is just ok for now and not
      configurable

    - support for builds, build artifacts & CI is disabled (we do not
      support CI (yet ?))

    - some internal defaults are just ok (e.g. where to organize
      directory for keeping repositories archives for downloads)

    - reply-by-email is not supported (yet ?)

    - we do not support LFS (yet ?) - just plain git hosting is ok for now.

    - Gravatar defaults are ok for now and not configurable.

    - Support for LDAP is disabled

    - Support for Kerberos is disabled

    - Support for OmniAuth is disabled

    - Satellites path is just /dev/null as we start from version where
      satellites are already non-existent.

    - Uploading backups to somewhere via GitLab's builtin mechanism is
      not supported - we'll use SlapOS native backup and resiliency for
      this.

    - Support for Google analytics is disabled.

    - Support for Piwik is disabled.

    - we are ok (for now) with default rack-attack git settings

/cc @kazuhiko, @jerome