Set up Renovate on Prospectus

We would like to set up Renovate on the prospectus repo: https://github.com/edx/prospectus

The one main wrinkle here is that prospectus is a private repo, and we will need to upgrade the renovate plan to one that supports private repos as well.

Currently, prospectus is using Greenkeeper, so this work is not super urgent, but we have been bitten by sudden and surprising dependency upgrades in the past and we would like to be better at maintaining this going forward.

Discovery ticket for this work: https://openedx.atlassian.net/browse/DISCO-1141