I've demoted the ObjectStoreUploader to a concern that is mixed in
the concrete uploader classes that need to store files in a remote
object store.

I've been working on making the local -> remote migration working
first, which has been trivial compared to the remote -> local one.

The current implementation is heavily based on side-effects which
makes the code brittle and hard to reason about.

The current approach is to store the `store` field in the correct
`Upload` model once a migration has been done. To retrieve the field
I use the `has_many :uploads` relationship, with all the paths that
a certain file may have `uploads.where(path: paths).last`. This as
the drawback of adding a database query for every upload lookup, but
I feel that the generalization of this behavior is worth it. We should
be able to optimize this down the road quite easily.