new API: rack-level endpoint that provides validation statuses

[karenetheridge]

We need an API, to be exposed in the web and shell UIs as well, which provides a rack-level view of the validation status of each device, including whether the device thinks it is the same type of hardware (from the report) as what we think should be living in that slot.

This might replace GET .../rack/*/assignment if its usecase is similar in the web UI and shell.

(this would also be available as /rack/:long_rack_name_with_room/... as well as /room/:room_id_or_alias/rack/:rack_id_or_name/..., as with all the other rack-level endpoints.)

blocked on validation engine rewrites, and some database schema changes.