Papertrail should trigger CLA-Bot

When opting to use the papertrail, there is currently no trigger for CLA-Bot to revisit open MRs and comment on contributors having (or not) signed the CLA. I think this can work via a webhook (or downstream pipeline, probably much easier) from store -> playground.

Need to make sure that if a trigger is used, the event or payload are not discarded...

If using a downstream pipeline, this should be much easier. However, this cannot be setup by the installer. Well, actually, since the installer has write/read_repo token, it could be done...