Create a "destroyed Milestone" event and keep Milestone events around in the DB for posterity

Also fix issue where destroying a Milestone would cause odd, transient messages like "created milestone" or "imported milestone".

Now if a milestone is destroyed, at least it will indicate in the activity feed even if it's not clear which milestone was destroyed:

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/c89cc8a0a9fa549deac433f17b890913/image.png)

Closes #2382

See merge request !1227