Implement change requests for change requests

[rolodato]

Is your feature request related to a problem? Please describe.

When working in an environment that has approvals enabled for feature changes, making modifications to an already-approved change request does not require approval. This means that users can ship unapproved schedule or content changes by editing an already-approved change request to have whatever value or date they want.

Describe the solution you'd like.

Trying to edit approved change requests should require approval, the same way trying to edit features does.

Editing unapproved change requests should not require approval, though we should reject edits if a user tries to approve an earlier version of a change request.

Describe alternatives you've considered

As a stopgap solution, it might be simpler to disallow editing approved change requests within environments that require approvals altogether. This would solve the immediate issue while not locking us in to a specific solution in the future.

Additional context

GitHub can require approval on pull requests, but by default allows merging changes that were pushed to a branch after the PR was initially approved without requiring further approval. An option can be enabled to change this: https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/managing-a-branch-protection-rule