Introduce an endpoint for bulk deleting users

[michaelklishin]

See #447. When a bunch of orphaned users in an environment with high user/vhost churn (typically when RabbitMQ is provided as a service) accumulates, it would be more efficient and operationally easier to have an option to delete them with a single request.

The endpoint can be e.g. DELETE /api/users/ with a single request body parameter, users, which is a list of usernames to delete.

[michaelklishin]

Per discussion with @albertoleal.