Move ChangesetEvents & Changeset state computation to enterprise

Created by: mrnugget

We should always err on the side of having more code in enterprise than in open.

This moves a lot of the actual code (not type definitions) out of internal/campaigns/types.go to multiple files in enterprise/internal/campaigns.

This is kinda related to #9101, because CalcCounts lives in enterprise and sharing code between open and enterprise can get tricky.