Make repo update schedule configurable

Created by: ryanslade

Currently we backoff based on a simple heuristic:

https://github.com/sourcegraph/sourcegraph/blob/320870aca21f1194446e0e9c034a8843c553ed22/cmd/repo-updater/repos/scheduler.go#L197-L202

Some customers would prefer if we could specify a fixed backoff, perhaps we can add optional site config to allow this?