Prevent concurrent fact check processing

Problem: we need to process fact check emails on a frequent basis. We only want one process doing this at any time. We run this script from cron, which (for resiliency) runs on every machine. Currently we are using a lockfile, which prevents concurrency on a single machine only.


This PR uses a distributed lock stored in Redis to solve the problem. There will be a change in alphagov-deployment to add Redis config.