Sidekiq JSON logging

This change enables writing the log output from Sidekiq into a JSON
format, which can then be consumed by Kibana.

This enables verbose logging for sidekiq which yields useful information
for investigating issues around scheduled publishing.
Merge pull request #1836 from alphagov/csrf-improvement

Better CSRF handling
Merge pull request #1834 from alphagov/sidekiq-json-logging

Sidekiq JSON logging