a8n: Remove published_at from ChangesetJobs
Created by: ryanslade
It can be assumed that once a ChangesetJob has been created it should be processed. Following this invariant, we no longer need the published_at field on a ChangesetJob.
ChangesetJobs are created from CampaignJobs in one of two ways:
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A Campaign is published leading to all CampaignJobs being created along with their corresponding ChangesetJobs.
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A ChangesetJob can be manually created by publishing an indivdual campaign job.
This part of https://github.com/sourcegraph/sourcegraph/issues/7782