"app setup" should create search indices if they don't exist

Part of https://github.com/cargomedia/CM/issues/169

At the moment we say people should create search indices manually on project creation (with `vagrant ssh -c 'denkmal/bin/cm search-index create'`).
I think this should be done in `app setup`.
We don't have a way of doing this *only if they do not exist yet* at the moment.

@kris-lab fyi