Allow govuk_crawler_worker RabbitMQ mgmt perms

Allow the `govuk_crawler_worker` RabbitMQ user access to the RabbitMQ
management API[1], giving it the rights to kill its own connections and
channels.

This is needed for CI tests for the GOV.UK Crawler Worker.

[1]: https://www.rabbitmq.com/management.html
Merge pull request #203 from alphagov/76261684-give-rabbitmq-user-management-access

[#76261684] Allow govuk_crawler_worker RabbitMQ mgmt perms