This commit refactors the direct external upload routes (get presigned
put, complete external, create/abort/complete multipart) into a
helper which is then included in both BackupController and the
UploadController. This is done so UploadController doesn't need
strange backup logic added to it, and so each controller implementing
this helper can do their own validation/error handling nicely.
This is a follow up to e4350bb966
