a8n: Rename previewCampaignPlan to createCampaignPlan

Created by: mrnugget

This is a breaking change I'm proposing to make understanding the API a lot easier.

I ran into the problem of having to explain what "previewing a campaign plan" means multiple times in the past few days. First, when explaining Automation-related concepts to Ryan and Kevin, and then when working on src-cli to add the ability to create CampaignPlans and Campaigns.

In the end I realized that previewCampaignPlan is simply not as clear as createCampaignPlan, which is what the mutation actually does: it creates a CampaignPlan that can then be shared, previewed and used to create a Campaign.

As I wrote in the CHANGELOG.md: I think we should use the time we're still in beta and behind a feature flag to make these changes.

I might have more changes like this coming up.