CORE-5832 Refactored Donkey's /arg-preview endpoint.

Renamed to match the metadactyl /apps/arg-preview endpoint.
CORE-5832 Refactored Donkey's /update-app-labels endpoint

Refactored to match the metadactyl PATCH /apps/:app-id endpoint.
Added medley library for dissoc-in calls in donkey.services.metadata.metadactyl.
Added a donkey.util.service/forward-patch function.