Make it possible to configure the "/clean" command to be disabled on a per-repository basis. This is useful for repositories that support that clean backports automatically are ready, but that doesn't want a user to be able to mark a pull request as clean (that the bots have deemed to not be clean).

If a user issues the "/clean" command on a PR in a repository where the "/clean" command has been disabled, then bots should reply with the message "The `/clean` pull request command is not enabled for this repository".