Strip private data before pushing to BigQuery

Created by: asdine

The LogEvent method is used to send events to both BigQuery and to the events table. We need to stop storing /~ sending the url and arguments column as they contain private user information and are not used for analytics anymore. Ensure no other part of the codebase is using these columns, through the EventStore) or through reading the table directly.

Ensure that we do not send these to BigQuery anymore.

Cf: RFC-358