[FEAT] Provide manual restart to avoid busy traffic

[tianhaoz95]

Also as of #769, the uptime of the app should be at 99%. If there is too much concurrent traffic, there is still a chance that some checks might not be picked up. Adding a manual way to restart the service for a certain pull request is a good way to navigate the bad user experience. Consider adding something like "@checkgroup restart" in the PR comment to tell the app to restart the run for the current PR.