uploads: Set X-Accel-Redirect manually, without using django-sendfile2.

The `django-sendfile2` module unfortunately only supports a single
`SENDFILE` root path -- an invariant which subsequent commits need to
break.  Especially as Zulip only runs with a single webserver, and
thus sendfile backend, the functionality is simple to inline.

It is worth noting that the following headers from the initial Django
response are _preserved_, if present, and sent unmodified to the
client; all other headers are overridden by those supplied by the
internal redirect[^1]:
 - Content-Type
 - Content-Disposition
 - Accept-Ranges
 - Set-Cookie
 - Cache-Control
 - Expires

As such, we explicitly unset the Content-type header to allow nginx to
set it from the static file, but set Content-Disposition and
Cache-Control as we want them to be.

[^1]: https://www.nginx.com/resources/wiki/start/topics/examples/xsendfile/

zproject/dev_settings.py

@@ -102,9 +102,6 @@ PASSWORD_MIN_GUESSES = 0
 TWO_FACTOR_CALL_GATEWAY = "two_factor.gateways.fake.Fake"
 TWO_FACTOR_SMS_GATEWAY = "two_factor.gateways.fake.Fake"
 
-# Make sendfile use django to serve files in development
-SENDFILE_BACKEND = "django_sendfile.backends.development"
-
 # Set this True to send all hotspots in development
 ALWAYS_SEND_ALL_HOTSPOTS = False