Send Content-Length from archive raw API

Created by: felixfbecker

Without a Content-Length header, we cannot show a deterministic progress indicator from language servers. We currently show MB downloaded, which actually caused some users to think it is downloading a repo to the user's disk and to uninstall the browser extension.

@slimsag mentioned we serve most archives from a cache so we could get a Content-Length from a stat in almost all cases.