Retain audit log events after deletion

[di]

In #6448 we realized that keying audit log events on the project/user IDs causes issues on deletion.

#6450 patches over this by deleting the orphaned events, but a more permanent solution needs to retain those events.

From chat:

We should ultimately key on users.username and projects.normalized_name. In order to key on the latter, it'll need to be changed from a column_property into a real (and unique) column, which will require a migration.