Add internal upload to external storage

This adds inline uploading capability to multipart handler.

API from GitLab exposes two pre-signed URLs (for S3 upload): 1. to upload the file, 2. to delete the file after the operation.

GitLab would generate these URLs with authorizing. One upload finish GitLab would handle these URLs and "COPY" file using S3 API to the valid path.

We still save file to disk in order to generate metadata.

It is to solve this problem: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9860#note_26534204.