Subscription management was only done in mount/3, meaning
navigating to a different record via handle_params/3 left the
old subscription active. A background update for the old record
would then overwrite the currently displayed record.
Fix: manage subscriptions in handle_params/3 (unsubscribe old,
subscribe new) via shared RecordActions.manage_subscription/2,
and guard handle_info({:update,...}) against mismatched IDs.
Claudio Ortolina
5f240fe573