From here on we start to authenticate uploaded file request before
serving this files in production. This involves allowing NGINX to
pass on these file requests to Django for authentication and then
serve these files by making use on internal redirect requests having
x-accel-redirect field. The redirection on requests and loading
of x-accel-redirect param is handled by django-sendfile.
NOTE: This commit starts to authenticate these requests for Zulip
servers running platforms either Ubuntu Xenial (16.04) or above.
Fixes: #320 and #291 partially.
efe8545303