Geo secondaries should correctly handle the migration of files to object storage

Per https://gitlab.com/gitlab-org/gitlab-ee/issues/4163#note_49783391

Currently, Geo secondaries will replicate an LFS object that is added to the primary unless the primary is uploading it to an object store. However, when the primary migrates existing LFS objects to an object store, the secondary is unaware of the activity. This means that the file remains on disc on the secondary, which continues to track it as an ordinary file.

The same logic is being rolled out to other types of files in the near future. We need to ensure that when a file of any kind of migrated to object storage, Geo secondaries take whatever action is appropriate so that they remain in sync with the primary.

/cc @smcgivern @jramsay