Implement the createCampaign mutation and necessary API fields

Created by: mrnugget

In order to implement the apply page properly, we need the createCampaign mutation. The resolver already exists and the schema describes what it should do, but it returns a TODO error.

What we also need to do is to implement the API as proposed in https://github.com/sourcegraph/sourcegraph/issues/12468 so that the frontend can decide whether to use createCampaign or applyCampaign.

• Implement the appliesToCampaign field on CampaignSpec (see https://github.com/sourcegraph/sourcegraph/issues/12468#issuecomment-667066717)
• Implement CreateCampaign mutation