Add chronograf dashboards to the backup script

The EFD backup script currently does not save Chronograf dashboards. They are not in the EFD or _chronograf databases. They are JSON documents that can be retrieved, for example, from:

https://summit-chronograf-efd.lsst.codes/chronograf/v1/dashboards

that would require the script to authenticate using the GH OAuth first, we can use GH sqre-user for that.