Add a comment trigger for rerunning failed Buildkite jobs

[DilumAluthge]

Summary

This pull request implements two different triggers that users can use for rerunning failed Buildkite jobs:

1. Reopen trigger: Close and reopen the pull request.
2. Comment trigger: Post a comment on the pull request that consists only of the following text: /buildkite rerun failed

Both of these triggers have exactly the same effect. When they are triggered, all of the failed Buildkite jobs will be rerun. The Buildkite jobs that passed will not be rerun.

Authorization

In order to use the reopen trigger, the user must be in one of the following three categories:

• Author of the pull request
• Commit permissions on the JuliaLang/julia repository
• Triage permissions on the JuliaLang/julia repository

In order to use the comment trigger, the user must be in the following category:

• Member of the JuliaLang GitHub organization (the membership must be publicized)

Notes

• With the comment trigger, the user can delete their comment after they post it.
• With this new implementation, we will no longer create "Retry" commit statuses every time someone adds or removes a label from a pull request.