Move some API endpoints over to admin/ namespace + add some request helpers

[ethanlee16]

• Moves user and viewing permission endpoints over to admin namespace (work may still need to be done on profile endpoints. I'm not sure what the intended use of those endpoints are quite yet, from an admin perspective.)
• Adds new request helper: routeByMethod to divide functions up by HTTP verb and use them to enforce some RESTful conventions.

Migrations

• Updates onUpdate/onDelete behavior of users <-> user invites. User invites now get deleted when users are deleted, but only when the deletion is performed at a DB level. No way to cascade delete when the foreign key is non-nullable prisma/prisma#2057 prevents us from performing a cascade delete on a non-null foreign key, so the connected entities are manually queried and deleted first for now.

CC: @erinysong