Post-review refactor Lookup API code. (#35)

The review discovered that the sync tool, when run on production, would never
succeed because the membership list of some groups/institutions was so large
that the API response exceeded the size limit imposed by API Gateway (due to
Lookup API returning detailed lists of members). This commit refactors the code
to fetch group/institution membership for each user and use that to populate
the membership set for each group/institution.