Disallow rate limits on user owned connections

Created by: ryanslade

Now that we allow any user to add an external service we can't limit only one rate limiter per code host which we currently do here: https://github.com/sourcegraph/sourcegraph/blob/c6f4b8b64886038d0a52927994c41985e4aa7a77/internal/db/external_services.go#L331

A quick fix would be to only allow site admins to define rate limits.

These limits are in place to limit the number of API requests we make out to specific code hosts and is probably not something we want to enable for users, especially as they can only connect to public code hosts.