Make help contact info on access denied page configurable

[ssciolla]

Is it okay to have these values be hard-coded? We could put them in the configuration, but I'm not sure how we would actually send to them to the client, since the user isn't authenticated and doesn't have access to globals. We'd need a public API endpoint with public data, but maybe that's more of a hassle than it's worth. Maybe it's possible to put the data in index.html and grab it (like we do in our Django projects), but we don't have a mechanism for putting configuration variables in that file yet.

Originally posted by @ssciolla in #316 (comment)

See the comment thread above for ideas including using webpack.DefinePlugin.