500 error on project deletion

[di]

Currently project deletion results in a 500 error due to #6339.

This is because the UserEvent and ProjectEvent models have a 'not-null' constraint on the project ID, and are not cascaded (deleted) when the project is deleted.

[woodruffw]

#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.