To make it possible to index the `from` and `to` fields with
Elasticsearch and other tools, we need the types to be the same.
Currently they are a mix of boolean and string values.

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/12599

This will add audit_json.log that writes one line per audit event. For
example:

{
       "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"
}