Add way to block IP addresses temporarily or permanently

[raxod502]

When triaging an ongoing availability incident, having https://github.com/raxod502/riju/issues/81 would be helpful, but once a root cause is identified we need to be able to restore availability until a code change can potentially be applied. For that reason we should have an admin tool that can be used to block traffic from a specific IP address for a duration of time.