[server] Provide 206 Partial Content for AVFoundation: AVPlayer

Summary: Temporary solution providing partial content to appease AVPlayer. Obvious issues to this approach: content needs to be fetched multiple times from DB, files need to be unlinked after they've been fully downloaded, validation/error handling, etc.

Test Plan: NA

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D728