The pattern of updating the model's `file_store` column after `file` was
saved repeated across many different models. In one case, the
`after_save` was missing, which led to a bug preventing object store
from working with the dependency proxy
(https://gitlab.com/gitlab-org/gitlab/-/issues/231563). Consolidate the
logic and tests into a single place.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/232094