Add new error-monitor-db dependencies & cron job.

Summary:
- Add manual setup steps for `client_secrets.json`, `bigquery_credentials.dat`, and alertlib's `secrets.py`
- Add a cron job for running `bigquery_update.py` every 15 minutes

Test Plan: `PYTHONPATH=/home/ubuntu/alertlib_secret /usr/bin/timeout 14m /usr/bin/python /home/ubuntu/error-monitor-db/bigquery_import.py -H "1s/0s: deploys"`

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: jace

Differential Revision: https://phabricator.khanacademy.org/D15088