Change all writes to go to
project_features.container_registry_access_level instead by
overriding the `Project#container_registry_enabled=` setter to write
to ProjectFeature#container_registry_access_level as well. This is
required because the project create/edit public API writes to
Project#container_registry_enabled.
At the next major version, we could drop support for the projects API
to modify container_registry_enabled. API users can instead modify
container_registry_access_level.