a8n: Implement CampaignPlan & ChangesetPlan in GraphQL API

Created by: mrnugget

This is part of #6085 (and RFC 42) and implements the GraphQL resolvers for the CampaignPlan and ChangesetPlan entities that were stubbed out until now.

There's nothing really tricky here: it pulls CampaignPlans and CampaignJobs out of the database and wraps them in resolvers so that the all the fields of in the schema work.

It's a massive amount of boilerplate.

Noteworthy is that I could reduce the number of Go interfaces needed by having the ChangesetPlansConnectionResolver, which is *campaignJobsConnectionResolver here, implement two GraphQL fields on CampaignPlan: RepositoryDiffs and Changesets. That gets rid of having to have an intermediate PreviewRepositoryDiffsConnectionResolver and doing that whole dance twice.

TODOs:

• Change the placerholder title and description (@sqs what should those be? Since the plan is not connected to a Campaign yet and the possibly hasn't entered title/description yet, we can't use that)
• Respect connection args in campaignPlanResolver.Changesets & campaignPlanResolver.RepositoryDiffs