Project Audit-Log

Description

Currently, there is no way for a project owner to track and reference major "actions" that are performed on his project (like deleting an issue). Fits in as a part of gitlab-ce#19313

Proposal

Implement an audit log on a project level that logs all major / significant actions (like deleting an issue).

Links / references

Requested by user:
https://twitter.com/sigmavirus24/status/811337290282766336