Cloud SQL snapshots as a Django pre-migrate signal receiver

As part of refining uis/devops/continuous-delivery/ci-templates#42 (closed) we discussed also adding a gcloud command to snapshot the db prior to any migrations being attempted to protect against failure.

This issue is to implement this on-demand backup requirment using the gcloud sql backups create command.

This issue has been refined and re-refined a few times! I've moved it from its previous location in ci-templates as the final design of this no longer requires any changes to those templates. Instead, we are creating a small Django library called cloudsqlsnapshot which will implement a Django pre-migrate signal receiver function to take Cloud SQL snapshots automagically when someone/something issues the standard manage.py migrate command. Pretty sweet, right!