Modernize db model relationships, auth checks

tidies up api v1 init module
updates legacy "dynamic" db model relationships to equivalent "write_only" relationships, and updates their usage in api resources accordingly
updates various other db model relationships to be more performant and clear
adds User.collaborators method, to get all users who work together on shared reviews
consolidates some is-allowed checks for api resources... this was more awkward than hoped for, so tbd on if this will expand
allows admins to delete users, and updates unit tests accordingly
un-skips citation import unit tests

datakind / permanent-colandr-back