a8n: Fix deletion of repos that are associated with Campaigns/CampaignPlans

Created by: mrnugget

This fixes #6727 (closed) by implementing the non-enhanced version as described in this comment: https://github.com/sourcegraph/sourcegraph/issues/6727#issuecomment-558510097

• The syncer now works again even if there was a Campaign/CampaignPlan that has a reference to a repository that's to be delted.
• Deleting a repository also deletes the related campaign_job, changeset_job, changeset and removes those from the campaign.
• Changesets remain on the code host when a local entity related to a changset is deleted.

What this does NOT include are the enhanced features:

• Closing Changesets on the codehost when closing a campaign
• Instead of removing changesets from a campaign upon repository deletion, they're shown in a "associated repository no longer exists" state
• Site admins are warned that changing an external service config will remove repositories that are part of a campaign