python: Sort imports with isort.

Fixes #2665.

Regenerated by tabbott with `lint --fix` after a rebase and change in
parameters.

Note from tabbott: In a few cases, this converts technical debt in the
form of unsorted imports into different technical debt in the form of
our largest files having very long, ugly import sequences at the
start.  I expect this change will increase pressure for us to split
those files, which isn't a bad thing.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/views/realm_icon.py

@@ -1,7 +1,7 @@
 from django.conf import settings
+from django.http import HttpRequest, HttpResponse
 from django.shortcuts import redirect
 from django.utils.translation import ugettext as _
-from django.http import HttpResponse, HttpRequest
 
 from zerver.decorator import require_realm_admin
 from zerver.lib.actions import do_change_icon_source