Add ability to manually reset a check run

If a merge run or try is in progress, there may be a need to manually reset the Checks page, in case the action gets stuck.

## Context

There was a case today where a merge run was stuck waiting for status from a CircleCI job that had already completed. I hit the reset button in the UI to let other jobs start running, but the stuck PR stayed "in progress".

When the reset button is hit, the merge bot should go through the PRs in that merge run and transition them to a failed state. Take care to make sure that any future updates to that check run should keep the check run in the failed state.

**Current workaround**: push another commit (either a new commit, or amend an existing commit)

Start by tracing the Checks-page reset action through the merge bot's handling of merge runs, pull requests, and GitHub check-run updates. Confirm how a reset can transition every PR in the run to failed and how later updates should preserve that failed state; done means the stuck run is released and future updates do not return it to in progress.

Written by the indexing model from the issue text.