Enable "Post comment when a pull request does not respect certain standards" for other `coqbot` users

[myii]

As discussed with @Zimmi48 in Zulip, it would be useful to have this feature available in our repos as well:

• https://github.com/coq/bot#post-comment-when-a-pull-request-does-not-respect-certain-standards

Ideally, it would be helpful to be able to configure the message in coqbot.toml but even having the message as-is would be a start.