endpoint: Add IP access rules to request

[erichanson]

Right now the Go server just controls who can access the server from what IPs etc through http.ListenAndServe() which isn't particularly expressive. How about adding a security layer in endpoint.request, or at least tables for configuring it. Maybe there's a slick Go library that does IP filtering that we could use and just pull filter rules from the database. We could do more expressive stuff than the server does now, and easily control it from the UI.