Validate spec contains changesetTemplate if steps are set
Created by: mrnugget
This fixes https://github.com/sourcegraph/src-cli/issues/406 and
https://github.com/sourcegraph/src-cli/issues/404 by making sure that a
changesetTemplate
is defined when steps
are set.
(Since we allow using only importChangesets
in a campaign spec, we need
to make this conditional on whether steps are set or not)