Unify path in NamespaceFileUploader with FileUploader & migrate old files

Right now we have different roots for NamespaceFileUploader and FileUploader.

FileUploader

def self.root
  File.join(options.storage_path, 'uploads')
end

NamespaceFileUploader

def self.root
  options.storage_path
end

We should unify that and use the path we use in FileUploader.

Because we already have some files in the current path we need to migrate old namespace uploads to the new location.

Product context

This is the follow up issue to do the long-term fix of https://gitlab.com/gitlab-org/gitlab-ee/issues/7009, after it was addressed in the short term with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21650.