Fix storing of events after creating a campaign
Created by: eseliger
Currently, this failed, creating new PRs until githubs limit of 100 open prs per head-sha errors the creation
errors:
13:37:17 gitserver | INFO command ran successfully, prefix: 10 github.com/sourcegraph/automation-testing , command: git update-ref -- refs/heads/test-gh-reuse-9 db905d0c2de0bddd46b8c8d2c8eef5c711720b15, duration: 7.486024ms, output:
13:37:19 frontend | ERROR UpsertChangesetEvents, err: pq: insert or update on table "changeset_events" violates foreign key constraint "changeset_events_changeset_id_fkey"
13:37:19 frontend | ERROR RunChangesetJob, jobID: 140, err: <nil>