Send one webhook request to discussions at end of mass-build pipeline run

[mbertrand]

Currently, the mass-build live pipeline triggers a separate webhook request to open-discussions for every published site. This may have been responsible for exceeding available redis memory on RC. It would be more efficient to send just one request at the end of the pipeline run, with with no site ids or maybe a list of all successfully published site ids. Either way, the API endpoint on open-discussions will also need to be updated to support this.