Add interest group participants endpoint

This will be a new endpoint attached to the organizations endpoint in the portal API. This endpoint will take an ID in its path as the ID of the Interest group to look up. From there, users will be mapped to their organizations, the organizations fetched, and then return a deduplicated list of those organizations.

To best utilize queries, we should add a new filter for Organizations in FoundationDB that allows us to pass a list of People IDs for whom we should retrieve organizations. This will reduce the number of requests we need to make and allow us to bulk process users. On the portal side, we should batch lists by 25 users at a time. This is to prevent issues with too long URL requests that would fail at runtime.

@cguindon do we want to add a second endpoint for this that retrieves all IGs and participant organizations? This would require us to do some loading caches likely to improve performance, but would also make querying the singular IGs faster if we do some smart management of our caching.