Add support for JSON logging for audit events

This will add audit_json.log that writes one line per audit event. Why?

• This makes it easier for customers to ship these logs to tools such as Elasticsearch, Splunk, BigQuery, RedShift for analysis
• Removes an immediate need to add an API for audit events, export to CSV, and provide more UI tools for querying
• Punts on database schema/management that we need in general for audit logs

Logs will automatically be rotated in Omnibus. Each event is generated in the format such as the following:

{"severity":"INFO","time":"2018-10-17T17:38:22.523Z","author_id":3,"entity_id":2,"entity_type":"Project","change":"visibility","from":"Private","to":"Public","author_name":"John Doe4","target_id":2,"target_type":"Project","target_details":"namespace2/project2"}