Partially addresses #47424.

For background see
https://apidock.com/rails/v4.2.7/ActiveRecord/Relation/find_or_create_by.

This is to avoid a mix-up with the existing concept of 'user
contributions'. See `User#contributed_projects` or
`Event#contributions`.

This is only relevant for migration specs. See
spec/support/migrations_helpers.rb.

This is due to the problem that the callback can be called while running
an earlier database schema version (for example during earlier
migrations). We work around this by checking the current schema version
and only track contributions if the table is available.

* Event without project
* Fail early on unexpectedly missing author

Closes #43460.