Change `log_audit_event` method to work with the new logging system
Description
At the moment we use log_audit_event function to record log the following events:
- Authentication made with LDAP, SAML and regular login
- Only in EE: Accept group membership, Leave group
- Only in EE: Group members creation and update
- Only in EE: Deploy key creation/deletion
- Only in EE: Project members actions (addition, update and deletion of project members)
We need to review this method and check how we can enhance it to record all the things we need to log as part of our logging efforts (https://gitlab.com/gitlab-org/gitlab-ee/issues/579).