Handle runbook links better

## Overview

We need to put some more thought into how we're handling runbook links. Currently assumptions are made that runbooks exist for a check and that the runbooks are named based on the check name. These aren't always valid assumptions. Also, it looks like the `runbooks.enabled` setting is not tied to anything. In other words, it doesn't appear that you can turn the runbooks link off in the notifications. So we need to look at that as well.

## Requirements

- Better handling of runbooks and their links
- Fix disabling runbook links

## Approach

TBD

## Security and Privacy

N/A

## Test Plan

TBD