Add contact email as a configuration setting

[jacobcook1995]

It would be good for each deployment to have a contact email defined, ideally in a form that can be easily accessed via the API. This could be added to the configuration, saved on the metadata server and made accessible through the API

[jacobcook1995]

@davidorme suggested that this would make more sense as a config option for safedata_server. With the following caveat, "But... there should be a docs section for setup that goes through those bits and makes sure admins set it up right." So, this is a documentation change rather than a code change