Add BigQuery ingestation of AppEngine logs.

Summary:
- Utility code to load credentials for BigQuery and run queries.
- Query for logs, parse them, and forward to the existing recording function.
- Can call the new `bigquery_import.py` as a script to load BigQuery logs into
  Redis
- When fetching errors from BigQuery, send an alert to HipChat if there are
  any new errors found
- In monitoring code, for rare errors we now check both monitoring and non-
  monitoring error logs for prior instances of an error before alerting
- Updated tests

Test Plan:
- Ran `bigquery_import.py -H "tom test room"` and verified that logs were
  fetched and new errors were sent to HipChat
- Ran `test_server.py` to ensure unit tests still work

Reviewers: jlfwong, csilvers

Reviewed By: csilvers

Subscribers: jace

Differential Revision: https://phabricator.khanacademy.org/D14956