`split_devices_for_user` returns a superset of the results of
`split_recipients_withhelds_for_user_based_on_identity`: let's reflect that in
the return types so we can start to share code.
Also, rename `split_recipients_withhelds_for_user_based_on_identity` to
`split_devices_for_user_for_identity_based_strategy` while we are here.
Richard van der Hoff
35ad5441d3