Permit a method of retro scanning without manage.py

## Background

Currently the only way to retrospectively scan objects in S3 buckets is via the `./manage.py` CLI tool. This blocks a 'set and forget' approach whereby we will have to manually execute this for retro scans. You can build it in a pipeline but that is still work required.

## Desired Change

Perhaps move/copy the retro logic within `manage.py` to a lambda which reads the inventory file to push messages to the analyzer SQS queue. It could even be triggered by when inventory files are created.

Relates https://github.com/airbnb/binaryalert/issues/164
Relates https://github.com/airbnb/binaryalert/issues/134