Adds jobs to crontab using backdrop-collector library

This commit introduces the backdrop-collector library for adding jobs to the
crontab in a similar way to backdrop-ga-realtime-collector.

1. Rename `backdrop.collector` package to `collector` to avoid conflict with backdrop-collector
2. Create a `collect.py` script as expected by `backdrop-collector`
3. Add post deploy hook to create the crontab in the same way as ga realtime

*NB*: This depends on https://github.gds/gds/alphagov-deployment/pull/310