model: make model entities frozen
we do not really need them to be mutable, plus we gain their instances now being hashable, so we can add them in set() for example.
Migrated from D2190 (view on Phabricator)
Staging instance, all changes can be removed at any time
we do not really need them to be mutable, plus we gain their instances now being hashable, so we can add them in set() for example.
Migrated from D2190 (view on Phabricator)