[Change Requests] - Let a creator edit and resubmit their declined change request

Description

Context
When a change request is declined today, it's a dead end. The person has to make a brand-new CR from scratch. We want the creator to be able to edit their declined CR's proposed changes and resubmit it, so a meaningful change doesn't just vanish on first rejection. The resubmit should land back in the original Slack thread and re-notify the original reviewers.

Task
Add a resubmitChangeRequest endpoint that reopens the creator's declined CR with edited proposed changes.

Acceptance Criteria

• Only the CR's creator can resubmit, and only when the CR was declined (accepted === false).
• Resubmitting reopens the same CR record: clears dateReviewed, accepted, and reviewer, keeps the same identifier and existing Slack threads, and replaces the proposed changes with the edited ones.
• A "this CR was resubmitted" message is posted in the original CR's Slack thread, with a button linking to the CR for review.
• Any reviewers who were requested on the original CR are notified again.

Start by locating the change-request API entry point and the existing Slack notification flow. Implement resubmitChangeRequest for the creator of a declined CR, preserving its identifier and threads while replacing proposed changes and resetting review state. Done means the original thread receives a resubmission message with a review link and previously requested reviewers are notified again.