Mass-build pipeline should not send individual webhook requests to mit-open/discussions

[mbertrand]

Description/Context

Currently, the mass-build pipeline sends an individual webhook request for each of thousands of courses to open-discussions/mitopen, triggering celery tasks there to update the search index for each course. It would be more efficient to send fewer webhook requests containing groups of sites to be updated: either all sites in a single request as the final step of the pipeline, or subgroups of courses as the final step within each batch of the pipeline.

Plan/Design

TBD