Backend: Remove the concept of a top-level "draft campaign".

Created by: mrnugget

From RFC 157

Backend

• Remove the Campaign.publishedAt field (and the concept of a campaign being a draft vs. non-draft/published).
• Remove the publishCampaign mutation.
• Add the publishCampaignChangesets mutation: publish all existing campaign changesets that you are permitted to publish.
• Rename retryCampaign to retryCampaignChangesets
• Don't publish changesets when campaign is created (user needs to explicitly publish changesets/campaign)

src CLI

• Remove draft option in src campaign create https://github.com/sourcegraph/src-cli/pull/217