upload: Fix browser caching of uploads with local uploads backend.

Apparently, our change in b8a1050fc4 to
stop caching responses on API endpoints accidentally ended up
affecting uploaded files as well.

Fix this by explicitly setting a Cache-Control header in our Sendfile
responses, as well as changing our outer API caching code to only set
the never cache headers if the view function didn't explicitly specify
them itself.

This is not directly related to #13088, as that is a similar issue
with the S3 backend.

Thanks to Gert Burger for the report.

zerver/views/upload.py

@@ -1,6 +1,7 @@
 from django.http import HttpRequest, HttpResponse, HttpResponseForbidden, \
     HttpResponseNotFound
 from django.shortcuts import redirect
+from django.utils.cache import patch_cache_control
 from django.utils.translation import ugettext as _
 
 from zerver.lib.response import json_success, json_error
@@ -40,8 +41,10 @@ def serve_local(request: HttpRequest, path_id: str) -> HttpResponse:
     mimetype, encoding = guess_type(local_path)
     attachment = mimetype not in INLINE_MIME_TYPES
 
-    return sendfile(request, local_path, attachment=attachment,
-                    mimetype=mimetype, encoding=encoding)
+    response = sendfile(request, local_path, attachment=attachment,
+                        mimetype=mimetype, encoding=encoding)
+    patch_cache_control(response, private=True, immutable=True)
+    return response
 
 def serve_file_backend(request: HttpRequest, user_profile: UserProfile,
                        realm_id_str: str, filename: str) -> HttpResponse: