Record an audit event for membership changes via API

Overview

As @brianwald pointed out in Slack, we don't currently appear to record audit events for membership changes made through our API. These most likely get recorded in api_json.log but we should ensure they're captured in the Audit Event UI as well.

Proposal

For project and group membership changes made via API, record an audit event associated with the appropriate project or namespace.

• We should capture the same metadata about the event as we do for other membership changes (we record membership changes made via the UI.