Add manual mode for the Rebase Branches

User Story

As a developer I want to prevent the bot from overwriting the rebase branch so that I can use the PR and fix up the branch to squash fixes manually or adjust the history manually.

Background

Sometimes we need to make additional changes to the PRs commit history before we merge. It would be good to be able to have the bot observe a label on the PR, `rebase/manual` that prevents it from updating the branch.

This would allow us to manually update the branches if we want to, for example, squash the history.

Steps

• Create a label on all repos that we care about for rebases
• Teach the bot not to push over the rebase branch when a PR exists and has that label

Stakeholders

• Cluster Infra

Definition of Done

• We can type /label rebase/manual on a PR to prevent the bot from updating the rebase until we have merged the current branch.

• Docs

• N/A

• Testing

• N/A