When deleting groups/users, delete corresponding group memberships, permission set assignments, etc.

**Is your feature request related to a problem? Please describe.**
If a user is deleted from AWS, there will remain a reference to that user within the group. Same for both users and groups which were assigned particular permission sets.

**Describe the solution you'd like**
Cascade delete foreign key references

No files, tests, or entry points are named. Start by locating the user, group, and permission-set deletion paths and the related persistence schema. Done means deleting a user or group also removes its group memberships and permission-set assignments without leaving references, with coverage for those cases.

Written by the indexing model from the issue text.